Assault vest instead of a bergen? The assault vest is an alternative to webbing, so I'm not sure why you'd want to wear it with webbing, although it may be possible.
What do you have against taking the bergen? If you have to take a reasonably substantial load (as I assume you are for 2-3 days) then the bergen is the most comfortable and suitable load carrier. Also, if you have a daysack, webbing and assault vest, you'll only be able to get the daysack off quickly, meaning that you end up carrying webbing and an assault vest if you need to dump kit quickly, when you should only have one of those two.