The Malteser Cross,
was registered as Vacheron&Constantin's symbol already 1880.
No, sure answer why.
Probably the most possible is,
that it's derived from the original malteser cross,
which is a functional part of the watch movement,
at least in older watches.
A stopwork function.
As you can see on this cross, that this actually has 5 arms, the usual were four, and one is always different.
That's the one that is stopping the mainspring from being turned to long, with risk that it would break.
This construction is situated under the barrel which contains the mainspring, and as you turn it,
it jumps over all the arms that are concave, but stops at the flat one.
That's the most plausible explanation, but no one knows for sure.
There have been several different crosses, but only quite small variations,
so you would not hesitate if it's a Vacheron Constrantin Malteser Cross or not.
Here are some different Malteser Crosses from Vacheron Constantin through the years,
so you could at least get some hint from when a cross could be.
I hope I helped you a little with the understanding of this beautiful symbol