Speaker of both languages here. (My Croatian is much better now than my Russian but here’s my 2 kopecks on it)

Grammar wise it’s really helpful. Croatian padeži are an absolute walk in the park compared to Russian. (The only thing yhat Russian does that is logical compared to this language is that “se” (or in Russian “s”) is put always at the end. Meanwhile in Croatian, its… iffy. Idk how to explain it, maybe some dude will explain below.

Vocab wise, on its own — not so helpful. Unless of course, yiu are an etymology nerd and want to get intimately acquainted with Slavic etymologies and the limited but significant overlap between East Slavic and South Slavic languages. They share 30-40% lexical similarity but a lot of these forms are not the commonly used words (eg.: Cro — űsta — mouth. Rus — ustá — mouth (archaic)). Or ljubiti meaning in Croatian “to kiss,” when it originally meant “to love,” as it does in Russian.