Can I currently prove it's a failure? No. Will anyone every be able to prove it? Probably not, unless SOE/Sigil release subscriber numbers that aren't inflated with the All-Access Pass which I doubt will happen. That being said, I say it's a failure for multiple reasons. One reasons is that I doubt Vanguard will break 200,000 subs. In this day and age, with the budget, credentials and publisher that Vanguard had, anything less than 300,000 subs is a failure. When other games with budgets a fraction of the size of Vanguard and no-name developers/publishers heading them have user bases that are close to Vanguard's, something obviously isn't right.
My second reason for calling Vanguard a failure is based on how much potential it squandered away. Two years ago, even after the birth of WoW, Vanguard was one of the most talked about upcoming MMOs for a variety of reasons. If you would've polled the MMO community at that time you would have probably seen a load of positive sentiment for the game. Now, only a month after release, Vanguard is being pretty much universally panned by critics and gamers alike. Not many are saying it's a bad game, it's just an amazingly mediocre game.
Now you can argue all you like that those don't necessarily make a game a failure, and maybe you're right. I'm sure their are 100,000 people out there who love the game and think it's the greatest thing since Everquest, but that could be said about a lot of games that are generally accepted as failures. I'd bet that if you really had a completely honest chat with McQuaid and Butler, they'd say the game didn't match their expectations either. So, is Vanguard a failure? In a common-sense view, it probably is.