cooking in a plastic bag without removing extra air, at 100c?
During sous vide cooking, it is not uncommon to place food in a zip lock bag and leave the top unsealed and hanging over the edge (so water does not get in) of the vessel you are cooking in.  It can easily be weighed down with some flatware, as you suggest.  There is no need to suck a vacuum and seal, in most cases. Alternately, you can use the displacement method to remove air and seal the zip lock.  However, your temperature is a concern.  Many vegetables can be cooked sous vide at or near 90C, but I don't think it is wise to go much beyond that, as zip lock bags can fail...and there really is nothing you want to cook that high anyway.  If you want to boil something, just place it in a pot of water and boil it.