[morzh]

MrFixr

I suspect that the problem will only be there if the glass is simply softened: it will start sucking the soft part in.

But I think the vacuum is not enough to really create an explosion if neatly scored.
So if you try one turn of some hot wire, or maybe both this and dip an cold water afterwards, or scoring first...something might work.

Now that I think about it, of you break the tip (neatly) the rest of the envelope might hold just fine. Then the rest could be dealt with.