Good luck ! there is some good asm tutorials and reference docs on snesdev. ASM is not as complicated as it might seems when you first look at it, perssonally I tried to learn it years ago but I thought it was too complicated and gave up but later, once I used geiger's snes9X debugger, it all made sense pretty quickly since you can use breakpoints and follow the code as it is beign executed. Atho personally I was used to "normal" programming before learning it.
Working with VRAM might not be the best place to start to understand ASM, I would suggest trying to do something else first but if your project is what drives you and you become OCD like me until it works, go ahead !
When I started learning, people on the romhacking.net IRC chat were kind enough to answer my noob question and/or direct me to documents that would answer them, so feel free to ask there if you need information.
This game looks fun, I hope you manage to translate it.
The game most likely re-use the VRAM adresses that are needed for the characters for other things when characters are not needed, you would need to find where the routine that loads the characters graphics into the VRAM is located, so you can know where the compressed graphics are located in the ROM and try to understand how the compression works or modify the routine that loads the graphics into the VRAM so that it loads your custom uncompressed graphics instead.