That's...actually not true at all...
On normal dungeon sure, on boss hive I disagree. Most classes aren't going to be able to reliably take on level 2 monsters at level 1 without a good item or glyph. Vampire is in the same boat in this regard, except that he's the sole class that can afford to quaff his potions that early.
And sure, Vamp can still pump out about 450 damage at level 5 with potions and fireballs and first strike, but not the 1300 we're used to from level 8 Vamps.
Maybe you won't be able to take on that level 10 boss, but you could easily take on that level 8 monster and gain another level, then take on a level 9 monster and gain another level, and now you're a level 7. The Vampire's combination of first strike, life steal, and a ridiculous number of fireballs means there's a long list of monsters that he can take on with a substantial level disadvantage.
Once he gains momentum, Vampire is home clear. It's all about identifying the highest level monster you can beat, then beating the crap out of it. Rinse and repeat until the highest level you can take on is "10".
Also, has anyone been able to clear the hive of all the monsters. Some of the tough ones are ones with high health or resistances.
Done it as Vampire, Transmuter, Monk, and Half-Dragon. If you get lucky and random weak monsters for your level 9's then this both greatly reduces the difficulty of taking them down plus gives you a great source of (relatively) easy XP. A level 9 goo blob is probably out of the question, much less on top of all the level 8's and 7's, but a level 9 bandit will probably give you an extra level and make those level 8's and 7's easy prey.