How would one say "freedom through strength" in Latin?
The word vis means strength and libertas is liberty/freedom. So would one say vis libertas?
I think you forgot the "through" in there. It's generally not a good idea to translate from one language to another word-for-word, as languages in general don't work that way.
I'd say the best way to translate the phrase wouldn't be "through", but "from":
Libertas ex vi
This gives the visual impression of liberty made possible out of [the use of/the ability to make use of] force or strength.
Other possibilities include:
Libertas ex viribus
This is making vis plural, which is when it's most often used of physical strength.
You could also possibly do:
Libertas ex virtute
Virtus doesn't really mean our modern "virtue", but rather encapsulates all the qualities that make up a good man, which include not only strength, but moral excellence and courage.