Hi Joop,
Check out West Marine, which, if your snap is the same as their snaps, has a tool that is real cheap.
If its the same as a standard carpet snap, I must have three tools I would be pleased to lend to you.
(Not Vise Grip style, but workable)
I think one of my tools looks like a BBQ tong, that can be squeezed by a Vise Grip.