Even as a Jugg I had constant problems holding aggro, especially early on with no taunts. Even when you finally get them (1 single target, 1 AOE), the cooldowns are so long as to make them effectively a 1 shot use in most fights. I just gave up using Vette once I had a melee DPS companion as she was constantly pulling mobs away from me. The only real experience I had using Malavai (healer) was when Lynk's Marauder and I grouped to tackle the last two planets (which gave us tank, healer, 2 x DPS). While he drew some aggro occasionally, I think it was primarily due to Vette once again dragging mobs off me, and then some of them aggroing on to Malavai (who would usually be right next to Vette). I really don't think a ranged DPS companion is a good combo with a melee class, but that's just my personal preference.
As a BH Merc DPS, I hardly ever found that Mako (healer companion) drew too much aggro. Generally it was only the occasional boss fight where it became an issue. I was usually outputting more than enough damage to offset any threat she may have generated. Plus the use of AOEs as an opener ensured I would aggro the entire group against me straight off the bat.