No matter what language you're learning, and no matter how complicated its grammar seems to you, one thing holds true for any human language you might study:
There is hardly any grammar.
How so? Go into a bookshop or library and compare the size of the biggest grammar book with the biggest dictionary. And don't forget that big dictionaries are printed on thinner paperstock than grammar books.
Juan Kattan Ibarra's Modern Spanish Grammar has 472 pages, and is pretty comprehensive.
Collins' unabriged Spanish dictionary has a whopping 2208 pages, in a smaller typeface than the grammar book and with a printed area approximately equal to two pages of the grammar book, and formatted to reduce white space to an absolute minimum. In terms of raw text, a comprehensive dictionary is about 20 times as big as a good grammar book.
On top of that, each grammar point in a grammar book needs a couple of pages of explanation and multiple examples, where a word gets a couple of inches in each half of the dictionary.
Really, there's loads of words in any language, and hardly any grammar.
So why not get the grammar early on? It's quick and it is universally useful. Very few people can pass a single day without using the majority of the verb tenses and noun cases available in their native language.
Vocabulary is a different matter. When did I last say "robot"? I can't remember. "Lamb"? Roughly four weeks ago. "House"? About 2 weeks ago.
So if I study vocabulary, I not only have an almost never-ending task (the dictionary I mentioned above has 315,000 references), but I also find myself unable to remember words because I don't use them enough.
But if I study grammar, I can cover all the basics really quickly, and those basics can be used every single time I have a conversation, and they will stick.
The best part, though, is that once you know grammar, learning words is easier, because you can use them and understand them in various natural contexts, because grammar can change both the form and meaning of words.
Right now, I'm mapping out the grammar of Polish in order to teach it to myself and a friend, and when I'm done, I expect to know less than 50 words. But I can learn more words later.