Beast Farm is horrible on Warlock. Tried it on mine (58 lvl). Garden of Best is also quite horrible.
I wouldn't do VoS, personally. Too much running. Treat a Summoner as if it were a melee toon. VOS is balanced for range classes. At the 66+ levels you will be doing WAY too much running up in the back (lower levels is fine for the most part, from ~60 to 66 or so).
TOI, Giant Cave, Catacombs (blech!), DVC are all some good and challenging places (except cata/necro on the challenging part, and somewhat exception on the good also :P ).
DVC and TOI are especially fun. I am going to try Giant Cave soon duoing with my roomate's ES. He needs SP and I wanna go wreck a couple rooms with the two summoners there anyways.
Warlock buffed with out of party SE & Prophet. Although didn't bother with the SE buffs so often.
Setup the out of party prophet with a variety of macros that I could hit to target a group mob.
The macro was set up to do a 1 damage skill attack and run back to me in order to pull a group I was not attacking to nearby in time to switch and attack so no down time.
And another macro to run to a mob that was aggro and delay and return.
Prophet was much higher so it did not ever sustain threatening damage.
That way I could effectively do a 'next target' attack for the entire time.
Even with moderately rewarding XP mobs, if there is only a split second in down time between mobs, it can produce very effective XP over time.
(Where possible, it can be very useful to use a 'boxed buffer' with even basic macros to increase your effectiveness. A waste to just buff every 20min and/or give occassional heals)