Could this be the fanfiction you meant?
Identity, by 'highbrass'
Rated: M, English, Adventure & Romance, Harry P., Fleur D., Words: 167k+, Published: Jul 13, 2005
Harry went on a journey, during which he found a destroyed city (i believe it was an Elven city)
A city, the most brlliant he had ever seen, lay in ruins. Smoke still rising from burning buildings and the tale-tell flash of shattered wards. Making his was though the destroyed city he couldn't imagine the grandeur it polished white stone and diamond all interwoven wit the forst itself. [...] A city of elves. The girl was one of the last of the race of High Elves left on the planet. - Chapter 5
He restored the city and its people and formed an alliance with them.
His grief turned to rage as he thought of what could have destroyed such a skilled and aged race. Then without caring of the consequences he removed the time turner, [...] - Chapter 5
On this part I'm not sure if that would work under restoring the city. Since he did technicly like restore the city, by going back in time.
In the city he received an enchanted weapon and was trained by them.
"I was hoping you would consent to train me in the elfin ways of fighting if you had the time to spare." Harry asked. "Of course" [...] The finished product was a weapon so powerful few could wield them effectively. - Chapter 5
In this part he learned over the years how to smith his own weapons and how to enchant them with the help of the elves. While it's not the same as just receiving an enchanted weapon it's still may be the same thing because he learned from them all these years to create this weapon and it was there rescources he used to create this masterpiece.
here was some sort of time barrier around the city (eg 10 years in the city = 1 year outside).
As you have saved mine and my families lives today as well as many of my people it would be an honor to host your stay here as long as you would like. I would like to add that I could change the wards surrounding the city let time pass slower inside than it does outside. You could spend years here and only a day pass in your world. - Chapter 5
Later in the story he saved a young vampire from being raped by a group of vampire hunters
He dashed in and out, down small alway ways, all the time sobs getting louder and louder. The site he came upon as one of revulsion. Five large men carrying multiple weapons were laughing as another was removing a woman's clothing; [...] "We've been sactioned by the Vatican to hunt and remove her kind from this land. Leave now or I will make you!" The elder ground out clearly angered by this boy's idiocy. 'So it was a vampire that these men had down. [...] "This motal saved my life as well as that of the council." - Chapter 3
This may be one of the reason why this isn't the correct one since this place before the elven chapter. But everything else would work fine here
is introduced to her clan and convinces them to join him.
"Any firther attacks on the Vestic clan and I will kill all of you personally" [...] After arriving back in the castle, the Vestic clan held him as a hero. - Chapter 3
This was one of the events where he saved the whole clan and because of this he got the whole support from this clan for anything he requires.
There wasn't really a scene where he got introduced to the whole clan but there was only when he got to meet the leader of her clan.
"I can be sure because I am the leader, I am Markis leader of the Vestic Clan for the past 1500 years. - Chapter 3
Also he is more powerful than usual, he's a human (no vampire or other)
He is more powerful than usual, since he received all this training from all the different races which he visited and learned there magic. He's in many ways way stronger then what he originaly was in the books.
