When travelling from place to place I keep getting this Vampire and his thugs,no matter what I do I can't kill it. I have got around it by casting invisable on the party otherwise I'd never get any quests done.
Any tips for killing him?

Have Clerics cast Negative plane protection. That stops level drains, and, even though it lasts a short while, can give you the precious time you need to deliver the killing blow.

Thanks,I'll give it go :)

If you have anyone in your party who can use axes and a couple thousand gold coins to spare, buy Azuredge from Bernard in the Copper Coronet. It is a ranged weapon that can insta-kill vampires, (and if they can't touch you they can't level drain you)and one of the easiest ways for low level parties to deal with the vamps.

and in general you need weapons of +2 enchantment or better to hurt them

I think traveling during the day will negate the vampires, i dont think they enjoy sunlight much

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You could use daystar, which can be stolen from a litch in the bridge district. you dont need to fight him, just take the sword and do a runner. then use its sunray ability.

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Daystar, and its accompanying lich, may be found in the City Gates district, within the Crooked Crane.

whoops sorry my bad

I've also found the cleric spell Holy Smite very handy when dealing with vampires. Of course, this should not be used if there are any evil "bystanders" or party members within range. Otherwise, heed the advice above. If you're still having trouble, don't walk around at night until you build your experience a bit.