Today I removed all the vinyl sacs from my pens and I didn't find any damage, which is good considering some of these sacs have been in a pen for about six years.
Right now the only place to buy pure silicone sacs is:
http://www.vintagepens.com/catill_nibs_parts.shtml (no affiliation)
They are only available in limited sizes right now, but the proprietor is working on making other sizes available.
You might also want to watch this:
|Suspect Silicone Sacs: removed from my pens, the one on the right looks suspect to vinyl deterioration|