What kind of keyboard are you using, and which Garritan Library?

I ask because some MIDI controllers might have a way to assign aftertouch to a slider, knob, or control wheel even if they do not have an after-touch pressure strip under the keys.

Some instruments in some Garritan Libraries might also provide alternative ways to control the vibrato intensity, but just not have it well documented.  We can sometimes find hidden or undocumented controllers by having a look at the sfz instrument files themselves of your Library's installation.  Files in the Garritan Installation with an sfz extension are simple text files that tell ARIA how to map out and play back samples.

As for the vibrato 'speed':  for instruments that support it, it should already be assigned to a normal CC event.  Look beside the knob for it in the "Controls" Tab of the ARIA Player.  I.E.  In JABB3, it's assigned to CC17.

As a last (and unofficial) resort, you can back up a particular sfz file and edit a copy so that it uses whatever unused CC you like to control vibrato intensity.  This requires a bit of learning about the sfz codec, but it's not terribly difficult to master, and there are experts on the ARIA Engine and sfz instrument building who can help you over at the Plogue user forum ( http://www.plogue.com/ ). 

In this case I believe the opcodes you'll be interested in are all internally assigned to cc129 (for after-touch) inside the ARIA Engine itself.  They'll probably look something like this:

lfo06_pitch_oncc129=1200  lfo06_pitch_curvecc129=12
lfo07_volume_oncc129=-1.5

Since MIDI only lets us to go from 0 to 127 with our typical programmable continuous controllers (CC)....we need to change it to respond to something available that is within our MIDI specification range.  In this case I'll go for CC16.

lfo06_pitch_oncc16=1200  lfo06_pitch_curvecc16=12
lfo07_volume_oncc16=-1.5

Note most instruments will have these sorts of controllers defined more than once throughout the sfz file [or use an include statement to bring it in from a different file] (for legato, key-switches, etc.).  I.E.  If you use a keyswitch or stomp the legato pedal things jump to a new <group> definition in the sfz....where any fresh lfo input is still valid.  It's also important to realize that the same controller might make adjustments to more than one lfo (or other parameters) as well, so browse the entire sfz and any potential 'include sfz files' to double check that nothing was missed.

Personally, I've tweaked out my Garritan Libraries via the sfz files a bit over the years to my personal needs/tastes, and have found it to be well worth the trouble to learn how.

Sorry I can't give more specific step by step instructions at this time.  I'd need to know exactly which instrument(s) and from what Library to attempt to offer up a 'specific' example for your case.  What I can tell is that in brief personal experiments thus far is that it can be done, and quite easily, and here are some 'rough examples' of how it can work.

Example that took about 2 minutes to cook up:

1.  I edited the sfz file for one of the Trombone Instruments from JABB3 that supports vibrato intensity via aftertouch using a text editor (Notepad that comes with Windows). 

On my system the trombone patch I chose to edit is found in:

"%SYSTEMDRIVE%\Program Files\Garritan\Jazz and Big Band 3\Instruments\03 Trombones\Trombone 1"

I chose to experiment with the "Trombone 1 KS" instrument from the JABB3 library since I know for sure that one supports vibrato via lfo.

IMPORTANT:  Before editing anything I made a copy of this on my desktop and renamed the original to "Trombone 1 KS.sfz.bak".  This is my backup in case I ever need to go back to it for some reason!  Windows will ask for permission before letting you rename this.

2.  I did a simple search and replace in the Windows NotePad Text Editor (found in the edit menu, or hit ctrl-h) for the following:

Find:  cc129

Replace with:  cc16

Then I clicked "Replace All" 

3.  I saved the file, and moved it to the proper Garritan Directory along side my old backup file.  Windows asks for permission before moving the file here.

Now on that particular trombone after-touch no longer controls vibrato intensity, but I can do it via CC16, which I have assigned to a slider/fader on my MIDI controller.

At this point, if I ever want to go back to the original setup, I can rename or delete my edited file, and rename the original (remove the .bak extension).

Using the three step approach above you could just back up the entire "Instruments" directory (the part that has the sfz files in it, NOT all the large samples) and very quickly convert things in an entire Garritan Library to use something different from after-touch in a universal manner.  A batch command line file/stream editing tool like sed would be perfect for the job. 

If I'd wanted  to take the extra time to keep after-touch compatibility AND be able to control it with CC16: in the same file here, I could have gone through and copied and pasted all of the "_oncc129" entries and duplicated them on the next line, but with the change needed for the new CC of my choice.  In short, I'd be 'adding' a new controller for some lfo parameters instead of simply 'replacing' existing ones.  That should in theory allow my new chosen CC while also still working with after-touch and the ARIA control panel.

Example:

I find this line in the sfz:

lfo06_pitch_oncc129=1200  lfo06_pitch_curvecc129=12

And copy, paste, and edit just below it so now there are two controller that can work this lfo:

lfo06_pitch_oncc129=1200  lfo06_pitch_curvecc129=12

lfo06_pitch_oncc16=1200  lfo06_pitch_curvecc16=12

I picked CC16 in this case because I don't think any of the Garritan Libraries currently use it for anything, it's also 'undefined' for 'general purposes' in the MMA MIDI Specifications, plus it's easy to remember since it's just below the CC17 used for controlling the 'vibrato speed'; however, you could choose something else if it's not already in use (conflicting) by the instrument, and better for your particular MIDI Keyboard (in realizing that some might not give you a way to change what sliders/knobs/etc. can send).  To double check that a CC is not already in use by the instrument, simply do a text pattern search through the file with a Text Editor.  I.E.  If you wanted to use cc72, then do a search (ctrl f) for it in the text editor to make sure it's not already being used for something else.  I'd also avoid ever trying to use anything lower than CC12 or higher than CC118, as they might not show up in an SFZ, but ARIA itself might like to interpret their use.  You will also want to avoid CCs 64 (sustain), 68 (legato), and 91 (reverb).

CC16 is used for 'aggression' in some Strings and Brass instruments. I'd suggest CC82 - 89 as a good range for user-defined CCs.