Well this sounds very famillar. It happened to us several years ago, right after I purchased our VX used and mildly abused (not maintained). The bad thing is.... i can't tell you at all what the problem is or was. I drained out as much atf fluid out of the trans and replaced with new and added Lucas Trans slip (don't remember the full name) atf additive.
I only remember it doing it twice ever, and it hasn't done it in 20K since.

How many miles on VX? last atf filter change?

couple of ideas..... keep us all informed... good luck