How to control the Rocket Powered VTOL?

There are a number of issues with VTOL. Biggest one is that the vectoring on engines becomes less and less efficient as the center of thrust is moved higher up towards center of gravity. If center of thrusts ends up above, you actually get engines actively trying to flip the craft over. So first thing is that if there is room at all, move the lifter jets down.

Second issue is that gimballing initiates when you fire a stage. If you just activate the engines via action groups, you get no gimball control. So fire all of your engines in one stage, then shut down these you don't need with action groups. When you re-activate these with action groups, they'll work as supposed to.

Finally, as fuel is used up, center of mass shifts. Your VTOL might start out perfectly balanced and hover steady, and then a few seconds later start turning over. Before you can properly react, the thing is flipping over. You have to make sure that fuel is being used up symmetrically around center of mass. Not an easy task, but that's the only way you'll get a good VTOL.