My son is 28 months old, and has the same problems. The only difference is, he is VERY vocal at times, but when he's around more than one or two people, or strangers, he just clams up. He is VERY intelligent, and can operate complex programs on his computer, like Paint, Notebook, opening actual web pages and playing music samples, etc. He is such a brilliant boy, and shocks me sometimes! Once, he actually recognized WORDS, instead of pictures, on the tv, and pointed, and screamed..."MOM! BOB-BOB!!!"...translation: spongebob squarepants is coming up next. He also recognizes numbers, and even understands phrases I had no idea he could even comprehend. He is very active, and always into something...eager to learn how it works. He likes to learn by doing, not by seeing or hearing. He does have a bad habit of pointing and saying "Dis!"(this!), instead of actually saying what he wants, until I ask specific questions, where the answer is not, as I had hoped, the name of the object, but simply "Yes", when I say the right thing. Given everything else, I am highly confidant his lack of vocabulary is perfectly normal, and probably some sort of personality quirk. I would say, just coming from personal experience, just let him grow at his own pace. I refuse to take my son to a therapist, because they are meant to fix problems...and I don't see him as having a problem. As far as I recall, I was a very quiet child myself, rarely talking, and always with my nose in a book or some craft. When I reached 3rd grade, I was evaluated for the Academically Gifted classes, and was found to have a reading level and vocabulary at college level. As a mom, you should be able to detect if there are any real problems. Just use your best judgement, and I'm sure everything will be fine. Just be sure to be there for him, and be supportive when he finally makes progress. Of course, if he just clams up until he's 8, you know you have a problem.
Good luck with your son.