Hi Skelly,
Unfortunately the xgo tool which I use for cross-compiling vikunja for various platforms does not seem to support freebsd. I’ve talked to the other maintainers and will try to provide builds for freebsd once support for it is ready.
I think the “hacky workaround” would be to use a Linux VM 
In the meantime, if you don’t mind compiling vikunja for yourself you could try something like the following (not neccarily from the nas itself, this should also work from your workstation):
GOOS=freebsd GOARCH=amd64 make build
(assuming amd64 as the architecture of your nas, if you’re using something else, you’ll need to change that.)
You should be able to run the resulting binary in a freebsd jail.
You will also need to host the frontend either through nginx or apache but I assume running either of these is a solved problem on freebsd. If you have any issues regarding the configuration of them, please don’t hesitate to hit me up.
Unfortunately I can’t provide much assistance for freebsd as I’m not using it myself.
Konrad