I noticed that final version of V-Ray 3.4 changes color mapping/gamma for materials from beta version as well as older versions. The old materials now seem too dark.

I think, it is because of gamma settings mismatch. Before, there was color mapping section in V-Ray settings and "linear workflow" option was set by default. Now this linear workflow switch is missing and it is controlled individually for each material. Therefore I think, you have to decide if you are going to work in gamma 1.0 or 2.2.

For gamma 2.2, you should uncheck "linear workflow" for every "old" material that uses texture in diffuse map. For old materials defined only by colors you should leave it checked - if you uncheck it you have to change rgb values according to sketchup material colors. For transparent materials you also have to uncheck "use color texture as transparency" e.g. for glass etc. All used textures should have set their color space to screen space (sRGB) and in V-Ray frame buffer make sure you have turned on the small icon "display colors in sRGB space". This way all should look correct.

Basically if any material has checked "linear workflow", it tells V-Ray that the material is defined in gamma 1.0. Textures have defined their gamma through color space drop down menu - when you choose screen space (sRGB), it means the texture will be treated like gamma 2.2, if you choose rendering space (linear) you are telling V-Ray it should be treated like gamma 1.0.

I also noticed that V-Ray now primarily uses diffuse textures from external files instead of textures saved inside SketchUp file and it is incapable of changing color space of those textures inside SketchUp file - it shows warning of missing textures.