Objectives: Find Mission Vao
If the Lower City was the frying pan, then the Under City is the fire. There seems to be a theme developing here, as yet again, you'll be accosted by hooligans as soon as you get off the elevator. Fortunately, they're easy to get rid of, by simply threatening their lives; or you can give them twenty credits for some Light side points. Afterwards, the rather over-eager Shaleena will give you a more hospitable welcome.
Explore the settlement; be sure to talk to Gendar, at the very least, and note that there is a healer within the settlement--you're going to need healing pretty soon. Rukil will give you Side Quest if you agree to find his apprentice.
Once you're ready to proceed, head to the northeastern exit of the compound. A villager has gone outside the gates, for some reason, and the gatekeeper refuses to allow him access, since he's being chased by a Rakghoul. You can either choose to help fend off the Rakghoul for Light side points, or insist that you're not risking your life and watch Hendar die to earn some Dark side points. After this little incident, you'll be able to pass through the gates without needing to speak to the gatekeeper.
Mission will track you down as soon as you step outside of the city and explain that Zaalbar was kidnapped by slavers. In yet another example of quid pro quo, she'll want you to help track him down in exchange for getting you into the Black Vulkar base, and she'll add herself to your party. If you're a soldier, you should start dancing now; you'll finally have someone with some decent security. Her awareness and demolitions skills will come in handy sooner than you think, as well. Equip her with some light armor, if you have any.
Now that you finally have a complete party, you can start looking around outside the city gates. Look hard, too--there are mines scattered here and there, especially around the downed escape pod, so let Mission recover them to add them to your inventory. You'll encounter a wounded and infected Republic soldier near the pod; if you don't have the Rakghoul serum, he'll change almost immediately. If, on the other hand, you've already found the serum, you can give him a dose to gain some Light side points.
There's a massive horde of Rakghouls guarding some bodies to the northwest of the city; one of these is Rakil's apprentice. If you intend to complete his quest, you'll need to search her body and bring her journal back to Rakil. If you can't bust through the Rakghouls, wait until you find Zaalbar and come back.
Heading to the east of the city, you'll run across a Sith patrol. Show your papers, then ask them about the other patrol that was reportedly lost. Once you're done questioning them, you can either leave them alone or slaughter them--if you do kill them, you get the experience, plus a Sith energy shield and a very handy set of motion detection goggles that you can give to Mission, or yourself, if you're a scoundrel, to boost awareness, demolition, and security. Proceed farther south from this point to find the Sith trooper's body with the Rakghoul serum on it.
Once you're done fiddling around, head to the sewers. There are two entrances to the Sewers; the entrance directly to the north of the settlement's exit is closer to Zaalbar's location, so go through that one.