i have a mac os x 10.5.5 and downloaded Vendetta online but what do i open it with?

Open VendettaOnline.dmg, drag Vendetta.app to somewhere on your harddrive and then start it from there.

if i try to open it it asks me to open it with something
and it is vendettaonline.dmg.torrent

leaves torrent ands keep only vendetta.dmg and not vendetta.dmg.torrent
rename it ( select it and apple+i)

Get a BitTorrrent (BT) client, it is a seed-based P2P file-sharing protocol (wikipedia article for further reading).

There is a small variety available to the Mac user, namely:

Original BT client (not recommended due to scarcity of features)
Vuze, formerly known as Azureus (whole slew of feature, but coded in Java, and badly: resource hog)
Transmission (the one I use, for a variety of reasons first and foremost the small CPU resource footprint)
µTorrent (still Beta, I think, and I am ashamed to say that I have not tested this one yet)

(list not complete)

All of these should be able to open the file and download the application. It should not leave a file named vendetta.dmg.torrent; if it does, something has gone terribly awry.

Edit: I'll add this to the READ THIS if Vendetta refuses to run on your mac thread.

Edit 2: Added.

Or just use one of the Traditional Web Download links at the bottom of the Download page and then install as normal.

But that'd be boring...

And some of us are very sad at the ratio we have for seeding the torrent files. Very sad indeed.

Yeah. I took them off the client again, might change that in the near future.