I think that the line:
_driver.Flip syncis the one which is calling a routine in the driver, which is probably platform specific, to do a Flip 0 or a Flip 1. The rest of the code seems to handle the Flip -1 condition. We'd have to look at the driver's Flip routine to see which parts of it are coded in BlitzMax and whether there is something there we can pull out to do just a vbl wait without flipping, otherwise it might involve dipping into C code or something.