Yup, that's something to remember. Language development at this point, if you're counting "expressive vocabulary", includes words AND signs. And since she does have quite a few spoken words, it's probably unlikely that there are hearing issues at play.
I have a friend with a toddler who didn't have any words at 12mo, this was odd for her since her previous 4 were chatterboxes. She asked her doc, who said he doesn't even ASK parents about speech development until they are 2.
In other words, anything under 2 is still within normal range.
My DD started a few spoken words around 15-16mo. By 21 mo she had maybe a couple dozen. It was around that time, shortly before age 2, maybe 22-23mo, she went though a CRAZY spurt, literally adding 3 new words to her spoken vocabulary every day!
We had never been concerned about her "later" development, since she had well over 100 signs, was obviously developing normally otherwise, and had great receptive language skills too. Plus, my little brother was one of those who didn't say a word until he was 3. Our parents were starting to consider some kind of therapy, when he started off with full sentences, all at once. And he grew up "just fine".
So while I agree that IF you're worried, it can't hurt to get an evaluation, it sounds like you probably don't need to be worried. 21mo is still young, is still within the normal range, and she seems to have good indicators that things are "normal" in terms of hearing and language comprehension.