Adding acid is scientifically incorrect as explained in this article:
http://www.popsci.com/diy/article/2009-01/shades-green
The article suggests that the best way to cook green vegetables and keep the colour is to cook sous vide at 100C or as near as possible for as little time as possible. It seems that the longer you cook for and keep warm then the more colour you will lose.
The best solution would appear to be to cook the vegetables individually sous vide at high temp until just done and cool rapidly.
If the purpose of having them in a bag together is about portioning them then you could do the individual cooking/cooling earlier in the day and re-bag then quickly re-heat for service.
