Either too high chile:vodka ratio or too long of an infusion time.
Try with lower ratio, or try infusing for far less time, say 30 seconds.
One of my favorite local bars does Thai chile-infused vermouth and they do a whole bottle of vermouth with about as many chiles as you describe. They do a traditional infusion, only about 24 hours. If a few minutes in an iSi is the equivalent to a few days or weeks of traditional infusion, you'll have to infuse on the scale of seconds, not minutes.
Also, Dave Arnold has a video on youtube about it, where he does jalapeños and tequila - it's also for not very long, and remember Thai chiles are much hotter
Edited by Hassouni, 05 September 2015 - 06:09 PM.