Having driven the Vanquish, I'll tell you that 700+ does qualify as "real high". In real life cars, 400-500 hp qualifies as "real high", even in a car as heavy as the Aston. I think you've fallen into a common trap, and are adding power but ignoring the rest of the car. Honestly, when I'm trying to go fast, power is almost the last thing I add. Let me ask you some other questions.
What tires are you using? Tires are the best and cheapest way to go fast. You should be using at the very least T2 Super Slicks. You can go all the way to T8 but those are very addicting, and they don't last long enough in races. I try to stay around the T4 range. If you're using Sports tires or Sim tires, get off of them right now.
Do you do any suspension upgrades? Start with the full race suspension. Lower the car by about 10% of whatever the stock height is, and remember to make the springs 10% stiffer to compensate. Then drive a few laps and start tuning the suspension. Search this board for tips on making adjustments.
As you've learned, more power isn't the answer if you can't control it.