I am currently Five boxing Vanguard. Only up to level 22 on that group.
Group is set up with Paladin, Cleric, and 3 Sorcerers. I am on a PvE server and have not tried PvP. I've been using Synergy + Hotkey (which has just been pulled from Beta) + G15 keyboard.

The combat in Vanguard doesn't work well with Multiboxing. With the frequency that you have to rely on timing for chains, reactions and rescues a mixed class group can be a real headache. As far as the basic commands, follow, assist, works well. In PvE the hardest thing I've attempted is Lord Tsang's tomb and have found with my five box group we struggle with the end bosses and usually wipe. For the basic mobs leading up to the bosses/elites I think my group can burn thru them with much more efficiency than a pick up group, but lacks the precision of a guild group.

Because of some of those limitations I've really had to adjust my combat style from normal play. Relying on basic attack command rather than unlocked chains can reduce DPS greatly and make aggro management more tedious. Hence the need for a cleric in the group. My original plan was 5 sorcerers, which I stopped at level 15 due to DPS issues with 4 and 5 dot mobs.

With a sixth member in the group from another player, either healer or another tank, my set up works very well. And in fact have been able to complete Lord Tsang's in record time. I prefer another Paladin in the group to off tank and keep things off the sorcerers.

Multiboxing has generally not been accepted on my server, lots of people two box, but with the ill feeling most Vanguard players have towards gold sellers, there is a general assumption that anyone with more than two characters under tow at one time, must be part of that lot. I am sure as I advance up the levels to higher end content I will have to hear it more frequently. Along the way I've been very professional and friendly to all and rarely will invite a pick up player into my group unless they "Get it". I never kill steal or run in front of other groups to avoid conflicts, and when possible aid a group with buffs/rez's/assists.

I don't know how long I will multibox Vanguard, it's losing it's charm for me. Might try a PvP server soon but can imagine the backlash I would get there as I think a five box team could own.