Generally there is plywood or OSB behind vinyl siding, which makes me wonder why no one filled all the other holes after they took the old signs down.
The weight is going to make a diff for sure.
If it's not to heavy, (the sign), toggle bolts might be enough.
All depends on the sign your making and what it is made of.
You can also pull back a piece of siding to see what is behind it, and there is a handy little tool for putting it back in place.
Investigate first Sherlock, and see if you can figure it out. Ask a construction guy or builder.
If you can measure and lag it to the wall, even better.