hwinfo --gfxcard
Then, search the output for lines containing words "Memory Range". There you will find upper and lower memory boundaries in hex. Convert hex to decimal, and you'll get what (I guess) is what you want. You might want to divide the number of bytes you get there by 1024*1024 in order to get number in megabytes...
I checked the above on my own system and the result was correct
Memory Range: 0xf0000000-0xf7ffffff (rw,prefetchable)
7ffffff --> 134217727 bytes 134217727 / (1024*1024) = 128 Mb
I have tried that on S3 Tri with 4MB VRAM and it shows: Memory Range: 0xf8000000-0xfbffffff (rw,non-prefetchable) and no prefetchable lines were found. I did as you say: 201,326,591 - 134,217,728 = 67,108,863 (=64 MB, which is simply not true) lspci -vv also shows 64 MB of non-prefetchable memory. But, of course ancient S3 Trio cards have no 64MB of VRAM.