I agree. Having had few problems with French and German in my youth, I thought I would enjoy learning Czech, the language of my new home. My age (53 when I started) and language-learning experience mean that I haven't really had difficulty with the grammar, which is something that defeats many English people who try to learn Czech. I just cannot remember the vocabulary! After a total of ten years' residence here, three attempts at formal lessons and a couple of 'teach yourself' books and tapes, I estimate that I know less than 250 words.
In that case, I'd (theoretically) suggest studying Grimm's Laws, and the rules of phonetic change from Proto-Indo-European to the Slavic and Germanic languages, as well as Slavic and Germanic philology (my area is the Latin languages, but hey).
I've found that recognizing the shifted vowels and consonants over the centuries allows you to "see through" the modern words and into their cognates in the other related languages, e.g. Vulgar Latin cl, fl, pl > Castillian ll.... llamar < clamar, claim, exclaim, call; llama < flama, flame.
The reason I suggest the philological approach is that even I, who have never formally studied the Slavic languages, can recognize etymologies related to English in mother Russian now that I'm trying to delve into the language a little (e.g. daughter, tochter, дочка; love, liebe, любовь [lyub].
It's worth a try, then you'll likely start remembering the vocabulary.