1. Can you disable vampires, or make them similarily harmless? I'm relatively new to DF, and I've yet to make a fortress that hasn't died in the first few years. Having vampires out for my blood is a bit much.
Setting up
advanced worldgen settings with the vampire curses reduced to zero
should make vampires not appear.
A less worldgen-parameter-altering setting, if you're on an existing world you really like, is to manually inspect every migrant's thoughts screen if they appear to have abnormally large skill lists. Vampires get kicked out of a lot of places, so they tend to have lists of past affiliations that go on for about a screen or so.
Of course if they're the monarch they don't get kicked out, but they announce their status
in their job description so you know what to expect.
Detected vampires can be walled in or pitted into harmless places, since they don't eat or drink they'll live forever unless disturbed.
2. Is there a way to find a location with a 100% chance of having Iron?
Um... change all metals to iron and embark somewhere with metals? (Actually, even that probably wouldn't work)
There's no guarantees with iron - it's most common in sedimentary rock, with only one of the three iron ores appearing anywhere else. Since most flux stone is
also sedimentary rock, this also solves two problems (iron + flux for steel).
Using the site finder to find a site with shallow metals and flux stone is possibly your best bet (outside of using DFHack and its prospecting function to find the approximate mineral composition of the area), just be careful that the site you're given doesn't fulfil these two criteria independently and/or in a tiny area.
The real best source of iron is its fourth ore, goblinite, found in most locations and usually delivered straight to your fortress by considerate citizens.
Alternatively, you could try trading with the dwarven and human caravans - this only works if they themselves have access to iron though! (Dwarves will bring steel even if they have no access to iron, because magic)