6GB is too little RAM to try to run two resource-intensive video applications at the same time.
If you must run them at the same time, set the RAM Reserved For Other Applications preference value high enough to give Vegas lots of RAM to work with.
http://helpx.adobe.com/after-effects/using/memory-storage.html#memory_multi...
---------------------------------------------------------------------------------------------------------
Todd Kopriva, Adobe Systems Incorporated
product manager, professional video software
After Effects team blog
Premiere Pro team blog
---------------------------------------------------------------------------------------------------------
