I don’t know about the 64MB. First step is to get a toolchain together that targets the CPU and try compiling one of the Rust libraries. Then you’ll need to cross compile sodiumoxide. At that point - no mean feat - you can try building all the components and try build the vault.
Setting that up is none trivial - but I did manage it for ARM, all except for one (or two) libraries which I was told would be fixed in due course. So I got as far as cross compiling the self_encryption example and running it on Odroid. I now have a Rust compiler on Odroid so haven’t touched that for some time, but I did document it on the forum: Cross compiling for ARM
I just followed what I could find in the internet and got some help here and there, so I can’t really help much (also time limited).