while the GTX1070 is certainly not optimal it should perform better than 20FPS from my experience. Did you check the resolution settings of the Vive Pro? By default it might render in a higher resolution than required. Please see our knowledgebase article on that issue:
A faster gpu is the most relevant component for VR, so go for the RTX2080 (super) in favor of the RTX2070 if you can. VRAM does not directly affect performance as long as all the necessary data fits in. This becomes relevant for really large projects with lot's of high resolution textures or in case you want to render high resolution images (4k and higher).