Lying to those close to you is the biggest danger sign. It's subconscious admission to yourself that you know it's a possible problem, but it's also statement of a belief that you don't think it's out of control.
Add in the fact that the empty bottles are vodka (people often think, erroneously, that, unlike other liquor, vodka use is easier to hide (the "no smell" rationalization), and add in the number of bottles that haven't made it into the dumpster, and it sounds like a serious problem to me.
Because unless those bottles are the aftermath of one or two mega-parties at home they suggest a problem that has been going on awhile. You don't just start consuming liquor in that amount at home from the get go. If you've just been dumped by a girl friend, you might go through a bottle once out of despair/anger/whatever, but you are then going to be hit by an almighty hangover that will keep you away for a couple days. And if you go through a couple at a single dumbass party, the empties are going to hit the trash soon after (in my experience the biggest slobs, of which I am one) don't let multiple empty liquor bottles clutter up the place.
My first thought was that you should gather up all the bottles, empty/half-empty/full, put them on the kitchen table, and confront him with them all at once and ask him to explain. I've been "enjoying" alcohol since I was 16, and not always in moderation either, and, I've never had 9 empty bottles of hard alcohol at one time. I've tried almost every variety of liquor at one time, many of them when I was your age, and it was the accumulation of decades to get a "liquor cabinet" full of bottles. Nine bottles of vodka is a lot of booze, and -- apart from "morning after a big party at the house" -- its going to be hard to explain away without admitting "maybe I do drink too much, maybe I do have a problem."
But the reality, how you handle it ISTM depends critically on the particulars of your relationship with your brother and with the particulars of his and your relationships with the rest of your family, common social circles, etc. Some brothers are really close, in which case bluntness might work best, others are not, and I don't know you're situation. You state that you are roommates, which suggests that you have been pretty close; and you refer to an alcoholic mother, which suggests that there are some other inter-familial tensions. Have these tensions ever gone violent, etc.
I guess what I'd probably do -- this has never come up for me personally having to make the decision how to confront -- is first talk to any family members/friends who know you both well, and compare notes and plot a strategy among you. Confronting the person is often necessary, but the reality is that no problem will be solved without the brother admitting the problem and committing to its solution, but the better the manner of confrontation fits the particulars of your relationships, the more likely it will succeed in moving him toward that admission/commitment.
I don't think the same thing works for everyone. But I do tend to believe that "getting it out in the open" is a necessary condition. Not sufficient, but necessary.
FWIW, that's my view. But, like wpr, I'm no expert.
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God.
Romans 12:2 (NKJV)