It is really important to ensure all traces of residue are removed so if you vac seal once you can clean that thoroughly before putting in into the non permeable layer. You also need to ensure everything is prepared as FLAT AS POSSIBLE, any 'bulges' in the letter can draw attention and result in it being singled out for extra attention. I suppose providing you clean the MBB to remove all residue then the first vac seal may be overkill. Have a play around with various techniques and see how flat you can get it and go with the best option I guess.