bootargs VRAM

I don't understand your question. Do you want to know the address range of vram or the size you can set for vram param in bootargs ? Minimum size of vram is the number of fbdev buffers and their size that you use in your application. Maximum size depends on mem= argument. The amount of memory available for Linux kernel + user space apps is mem - vram size where mem is the "mem=" argument in the kernel bootargs and vram size is "vram=" argument in the kernel bootargs.