Aside from the Oculus specific things, I really don't know, there is and was always some sort of annoying stuttering in VaM that I have not experienced in other games. Especially if you leave the menu open. If you close it, IMHO the lagging will be a bit less severe. Many users will now say that they never have experienced this with VaM, but in my oppinion this is something that strongly depends on how sensible someone is towards stuttering. Some will play VaM at 25 fps in VR and can't understand why others say it is too slow.
At the other hand, this is a somewhat normal behavior with VR. Moving your head around in VR will caus a constant rendering load. Your brain will "smooth out" some of the smaller stuttering, and so will maybe do some clever smoothing algorithms. Moving your head quick will cause a big jump that can't be compensated. In other games there are most of the time low poly objects and small textures that can be rendered quickly. VaM is comperably high poly and has dynamically generated hair that has to be calculated in realtime, aso.
Refresh rate: this is a somewhat complicated thing in VR.
It highly depends on yourself. At the beginning of VR some scientists found out, that a minumum of 80 Hz would be neccessary to not feel nauseous. With some training it may be less, or not.
-Less refresh rate ~= less fps = more nausea.
BUT: if a GPU cant render a frame fast enough, it will drop it. Missed or dropped frames will cause nausea, too.
-Less predifined refresh rate = less framedrops =less nausea.
Because of this, in modern VR headset, if the predefined refresh rate cant be reached over a certain time span and ammount, it most of the time will be cut by half (or maybe with some other algorithms) and a software will add doubled frames. Some users are even sensitive to this, but more important is, that an doubled wrong/empty frame is more obvious than a single one.
So it is still important to have a high framerate. Framerate and refresh rate is not the same, but there is a correlation.
This will lead to the effect, that sometimes a higher refresh rate that can't be reached and therefore cut by half, is better than a low predifined refresh rate that is cut by half. For instance: 120Hz / 2 = 60 vs: 80Hz / 2 = 40.
BUT: if even the framerate cut by half can't be reached, it would be even worse.
The best thing would be to lower the game settings to have a native framerate high enough. If the 70 fps in your case are somewhat stable, it is great. BUT: If you turn your head and the smoothing algorythm is kicking in, you will maybe see a more severe drop than if you would be on a lower framerate, allready doubled by the software. It depends on you.
You see, it is complicated. Sonds stupid, but best thing is to try it out yourself.
