Their increased cost adds to balance. You have to spend more to get what they offer, which in a lot of cases is only just as good as the Vulture anyway, and you risk far more by taking them into combat. They have far more negative aspects than the Vulture does. The Anaconda, Python and Dropship are all slower at top speeds; all four are less agile. Clipper and Dropship don't have good hardpoint placement at all, forcing you to use gimballed weapons (which really aren't fun against a small-profile, agile ship like the Vulture, especially when it's popping chaff and taking basic evasive action).
Top-spec shields on the Vulture are better than both the Clipper and especially the Dropship (which gets a measly 235MJ!), and only marginally worse than the Python. It does have a weaker hull than all of the four you listed, but makes up for it by being able to take so much shield damage and disengage from most fights when things are against it. You can slow the retreat with a Clipper or FDL (Asp can match speed but I don't think it mass-inhibits the Vulture), but that's really it.
It's a great ship, but there are few cons. When you don't have a hard time shredding the bigger, more expensive ships in something that you can run for 11m, I would personally say that's a little excessive. I bought one at its reduced price and basically made that back within a day. It just feels too good for what it costs right now, in my opinion.
It's not too hard to stay behind even player Anacondas, and when you can't, your small target and use of chaff can make it hard to hit you anyway since most Anacondas run some combination of gimballed and turreted weapons. Pythons are much more dangerous to the Vulture in my opinion, as they can move a little better and faster, and bring fixed weapons to bear, but even then they aren't too hard to deal with (they would go down much faster without the wonky power plant hitbox, IMO). With both of these, your biggest worry is less their firepower and more the number of shield cells they can pop during a fight. If you ever get hit by the huge hardpoint on a 'Conda, you're doing something wrong.
On another note, would anybody be up for a bit of PvP fun when I get back from my trip? I'm thinking Eagles, since they're inexpensive. If anyone doesn't really have funds, I'd be happy to finance it for you. Shouldn't take much to kit out and the rebuy cost is minimal. I figure after a huge journey like this, my combat skills are going to be quite rusty.