Adding more vRAM to a GPU card isn't going to do much if the actual GPU processor isn't very fast. It might help in some games playing in Ultra mode, as an example.
Plus adding more vRAM you might need to modify your GPU vBIOS to be able to access and recognize the extra added vRAM.
Also adding RAM sticks might interfere with other hardware in a tight PC case or with the CPU Cooler. Best way, in my opinion, is to have a GPU card where the vRAM chips are removable and upgrade-able like a CPU processor.
Also you need to consider if the GPU processor can handle more added vRAM.
I did read once where a User upgraded his GPU Card vRAM by desoldering the current vRAM chips and soldering higher density vRAM chips. But don't remember how that turn out if the GPU card work afterwards and if there was any improvement.