Erm.. Question?

[NATHAN_W]

So basically, I wanna use the "Thud" engine on my Shuttle but the "vector" uses the fuel for the thud, any way i can stop this?

[steuben]

The simple way is don't turn on the vector.

But it may not do what you want. You want the vector and the thud to have their own fuel tanks?

[GluttonyReaper]

If you right click on a fuel tank, you'll see the amount of fuel in each tank, and a green arrow next to each (one for liquidfuel, one for oxidiser). If you click these green arrows, they'll turn into red circles. This locks the tank, and prevents engines drawing fuel from them. If you do this at launch with the tanks you want to use with the Thuds, you can unlock them again when you need them and have turned off the Vectors  

[Bill Phil]

If I remember correctly, there's a way to mess with the fuel flow... I think it's in advanced tweakables.

[p1t1o]

You could apply a limited throttle in the VAB to the vector. This will affect the ratio that the two engines pull at max throttle.

[Vanamonde]

This question about how to play the game has been moved to Gameplay Questions. 

[Superfluous J]

I would need to see your ship to know how to answer your question, but in general you must totally separate the vector and its fuel tanks from the thud and its fuel tanks, by some part that does not allow cross-feeding of fuel.

[bewing]

If you want them both running at the same time, then 5th Horseman is correct. If you don't want them running at the same time, then you should Shutdown whichever one you want to stop. Or lock the fuel tanks, or disable crossfeed on a decoupler.

Edited December 8, 2016 by bewing