Lowering the DSP buffer value will reduce the latency the Nebula brings. And with a DSP buffer very high it can go towards 16384 samples of latency, which some DAWs even don't properly handle .
You shouldn't have to make sure every track has the same latency, your DAW (if it's any bit of 'modern' descent DAW) will compensate all the tracks to be properly aligned. If you have one track with 16384-latency Nebula on it, and another track with a 128-latency Waves plugin, your DAW should make sure the 128-latency track gets delayed enough to match the 16384-latency track.
It should not affect how fast / smooth nebula switches between the vectors (but please correct me if wrong here). AFAIK that's what the Liquidity control is for, and on old programs you had a weird 'attack/release' which you weren't allowed to touch . These days those parameters are hidden from the main interface I think.
What cupwise is saying is that (for sound) it should have no bad impact to set the DSPBuffer very high, but the latency will be awkward to work with if it's very high, so you might want to finetune the DSP buffer a bit for your system.