WRT the staff of the magi, use it. It's now classified as an artifact, so you won't get anything with analyze dweomer, but a carefully worded divination should be able to let you know whether you'll depower it by using a few high-charge powers. Once you've used them, ready actions to absorb spells cast by the vampires until you've replentished the charges you use and then use those charges again. You don't need to know exactly how many charges you have as long as you know that you can keep going through the same four to nine charges again and again.
What other items do you have available to you? My advice is that some of the best other items for you to acquire would be:
A phylactery of undead turning--with that, the cleric should be able to turn vampires fairly reliably unless they're individually powerful enough to have CR>2+your party level without being vampires.
A lesser metamagic rod of extend spell--magic circle vs. evil? Cast it on everyone and make it last twice as long or keep it going all day. At 11th level, a single casting lasts 1:50 and an extended casting lasts 3:40, so you'd need three rods of extend spell and seven slots devoted to the spell to keep it up all day. That's a pretty painful investment, but if you don't expect combat on any given day, it's probably worth it to have the spell up when you're attacked.
A silvered undead bane weapon. At a +1 equivalent enhancement, bane is a no-brainer when you know the type of your major enemies--that's what it's for.
Other things:
One challenge of dealing with a vampire is dealing with it permanently. When reduced to 0hp they turn to mist and float back to their coffins. Now, you could follow them back to their coffins because mist form is rather slow, but if they're clever, you might need gaseous form for that and if they're clever, they will go via a route that is very dangerous to you (trapped or inhabited by other foes). So, you can't count on being able to follow them to their coffins and stake them. It's nice if you can but don't count on it.
What other was are there to kill them?
Assuming your DM doesn't use the dying of the light rules (the stake rules are from that dungeon mod if I read the reviews correctly), you could try having a deathwarded character grapple and pin the vampire while someone else stakes it. This, however, is unlikely to work. First, the DM might rule that staking is like a coup de grace and the vampire is pinned, not helpless. Second, a clever vampire would just turn gaseous to escape the grapple. Third, vampires are dangerous grapplers and could use blood drain on the grappler. If you're going to use a stake, you'll need something like the halt undead spell. Odds are good that the vampires will save against it, but you only need them to fail once....
Another method for killing the vampires would be to expose them to sunlight. If inside a building, a well timed disintegrate aimed at the ceiling might manage this. Be careful not to bring the roof down on your heads, but it could still work. In a pinch, stone shape could perform the same function. If you have a scroll of sunbeam or sunburst, those would work too (though you'd need to get the vampires to fail a reflex save and they're pretty good at those--bring some recitation spells if your DM allows Complete Divine/ Defenders of the Faith material).
A final method for killing vampires would be immersing them in running water. There are a number of ways this might be managed if you engage them near a stream or river. Grapple--it would take two consecutive grapple checks--one to grapple them and one to move them--in order to pull it off. Or you could bull rush--using telekinesis or Bigby's forceful hand in order to get them into the water. At the DM's option, a quickened grease spell on the ground underneath them might make it easier to bull rush them.
Finally, if the vampire is reduced to 0hp and then kept away from his coffin long enough, he will die. You might accomplish this by catching the gaseous vampire in an otiluke's resilient sphere and then building a ventless box around it using wall of stone and stoneshape spells. A hemispherical wall of ice might serve a similar purpose but wouldn't be able to keep the vampire from escaping through the floor so only try it on nonpourous surfaces.
Even following these strategies, a likely result is that you'll destroy a few vampires and others will escape to come back for more. What can you do if you don't manage to destroy the vampires permanently?
1. Escape. This is when the magic circle and the vampire's weakness vs. holy symbols comes in. If everyone stays in the magic circle and presents their holy symbol to the vampire, it can't make melee attacks and it can't dominate party members. At that point, you can back up to a secure location and rest.
2. Fortify yourselves. Vampires can't enter buildings without an invitation. So, sleep in a leomund's secure shelter. The vampires can try to dispel it (and will probably eventually succeed, but they can't enter. If you can find a real building that is not public to sleep in, that's even better. Temples are arguably public, but they are also usually hallowed--meaning that you have an even better chance of turning them and the magic circle vs. evil effect will protect you from domination.
3. Attrition. When vampires go gaseous, they tend to take their loot with them. But they can't take it if they don't have it. Consider disarming or sundering their weapons and holy symbols, grappling to remove their periapts and cloaks, etc. Living in a dungeon full of treasures, they will no doubt replace some of it, but the replacements won't necessarily be of the same quality. Plus, you get loot every time you fight them.
Also, consider permanent duration spells like bestow curse. It's possible that the vampires will have a means to remove them but it's also possible--especially if you wax their spellcasters early on--that they won't. When they come back, they may still have the penalties you inflict on them.
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