If you know they are gonna vulch and extend, then position is your friend.
Get a touch of alt, and a bit of energy, and position yourself so when they make their pass, you are diving in on 'em and end up right behind them.
Takes some practice, but I can catch most anyplane like this. The "trick" is to be ready for you shot, and hit 'em.
In other words, you know where they are gonna be, so position yourself to be there too at the same time. Sure they are doing 500kias and you are doin 250 BUT if you time it right, you end up D100-400 on their 6 and for a second or so you have a shot.
I can't count how many planes i've gotten to do a stupid Energy bleeding move when they were twice as fast as I was when I suddenly showed up 200 back on their 6 guns blazing. The smart ones keep going, but if they are flying the way you describe, odds are they none to smart.
I catch many a vulcher this way. (works with cherry pickers too)
Position, position, position. (my squaddies get sick of me preaching position)