The problem is that the VSTi is responding to note-off commands, which is normal for most instrument samplers. What you need is a dedicated drum sample player which just triggers the sample and lets it play to the end. There are lots of good free ones available - try this for starters.

www.vstplanet.com

The only other solution I can thinks of is to edit the midi notes to increase their duration. (Tedious)

ROG.