The AC/heater (HVAC) system may unexpectedly switch to the defrost mode when accelerating. This system is operated by engine vacuum and should be inspected for any vacuum leaks if this problem develops. There is also a revised vacuum check valve available to address this concern.Google+
Car Problem Reports
Dodge Viper HVAC Air Flow Switching to Defrost When Accelerating
Dodge Viper Problem
Yes this is due to the lack of intake vacuum during accelleration. HVAC system has a fail safe mode of defrost on vacuum operated HACV systems, to insure the capability of defrosting the windshild no matter what, goverment mandated. This issue could be corrected by installing a large vacuum resivoir in series with the vacuum tap at the manifold or add a oneway check valve in the lines to prevent HAVC vacuum from deceasing during acceleration events, effectiveness is based on HVAC system bleed down design some have slow, some have fast, and some have function change bleed down stratagies. My vehicle works with a check valve inline.