In such cases it is wise to made full backup and set it in test forum. Then check it all in test environment.
Also - you can always install 2.x again - you will only loose your cache content, but it will fill again.
Also vBET 3.3 should support all places which was supported by 2.x - in point 3.x has more possibilities, there was nothing removed - only added
And at the end answering directly to your question - booth versions of vBET have same ID, so it will not work. If you install 3.x without un-installing 2.x then it will simply override 2.x (with removing old cache tables).