I vanquished all of Ascalon in one setting last weekend. It took me just over 11 hours and I started from Yak’s Bend. I used SAB’s build, myself playing the 3rd Necro, and included Gwen for interrupts and quest requirements.
I completed Gwen’s quest “Then and Now, Here and There” which actually made things easier. The number of enemies in Diessa Lowlands was reduced and the Charr were hostile to all other creatures, essentially doing my job for me. The quest made little difference in Old Ascalon, however.
I took the path of Yaks -> Diessa -> Dragon’s Gullet -> Old Ascalon –> Eastern Frontier (and everything along the way). By caravan vanquishing from Yaks could bring a party of 6, which helped considerably.
I only had one wipe, when I was overwhelmed by mobs of Grawl in Eastern Frontier. They weren’t as bad as people have said, though it did take considerable effort and focus fire to take the Ulodytes down. Chests were few and far between, but I managed to get about 17k for my troubles after all of the selling. In addition, I only used 2 powerstones (once after the wipe and once to compensate for Alesia’s DP when she decided to tank several Charr). Ranks in the Ebon Vanguard title helped out a lot and I highly recommend taking the Ebon Vanguard Assassin Support PvE skill with you, regardless of your EV title rank.
I still have several areas to vanquish, in all campaigns. I’m not nearly as far as Kousei or as organized, but I’m well on my way to Legendary Vanquisher.