I'm not familiar with the Adesso tablets, but if you have no_ink.inf in your resources folder then both ArtRage and the other applications will be using the wintab tablet input system rather than Ink Services in Vista, which is generally the cause for different behavior between ArtRage and other apps here ( ArtRage by default uses Ink if available ).
The only other settings in ArtRage which relate to the tablet are the precise tablet mode toggle in the edit menu ( Edit, preferences, use precise tablet mode to on/off and restart ArtRage - I'd imagine that the Adesso would probably work better with this unchecked ). Check you're using the default tool settings by clicking the 'reset' button in the left hand tool panel, then see if the behavior changes. You can also set ArtRage to defaults by holding down the spacebar as it launches ( You'll know it's worked if it checks for updates immediately after launch ). Also, you can manually increase the pressure value using the pressure dial in the left hand tool settings panel if you haven't previously been doing so.
Failing that, I'd recommend checking your tablet software for pressure settings / profiles for apps and if the tablet software was upgraded from a previous version on Vista, doing a clean reinstall of the tablet software. It's possible that Artrage is receiving unexpected pressure values from the tablet software under Vista which could cause this sort of behavior, and the other apps use / interpret the pressure values in different ways.
I hope that's of some help!