First, go to RHS and check their database. You need to check both ways (A x B and B x A). Here is the link:
The International Orchid Register / RHS Gardening
If it has not been named, then you enter it in the show as:
Vandachostylis unnamed grex
(Van. Lou Sneary x V. vietnamica)
Since show schedules vary from show to show, I can't tell you what class to enter it into (first one that fits, if it is wrong, the judges can move it to the subsequent class that they think it should be in).