Yeah, its like what @haelgaan said. You could soft stack, but stacking is usually something to help healers, not DDs. As you don't have a healer, stacking is unnecessary.

I have been slotting Runeguard of Still Waters on my Arcanist when I run portal, and usually throughout vSE. Decent shield, nice little "OH S***" burst heal if you get caught off-guard by a mechanic, it applies everyone in your group in range of the skill, it does not mess up the tank's taunts (if done after the tank takes control of the boss and/or adds), and it runs on magicka alone if you are a stamina Arcanist, so it does not deplete your resource pool.

For the lightning portal, there are two things to watch out for.

The first is Sparks, which is effectively a 270-degree cone whenever the boss light attacks the tank. Yes, I said 270, not 90. Normally, a cleave cone will hit you if you're in front of the boss. But here, the Sparks cleave will hit you if you're in front of the boss or to the side of the boss. The only safe spot is directly behind the boss.

This is why most groups that run vSE will have everyone stacked tightly behind the boss for the lightning portal, so that they are clear of the Sparks cleave.

But wait, what about the AoE?! That's when the boss places an AoE on a player, and this AoE ticks for about 3K every half second (so about 6K per second) on vet non-HM (it's double that amount on HM). And yes, everyone in that AoE will take that damage too. Which is why there is an urge for people to disperse when that AoE comes out. But dispersing means that you run the risk of people getting hit by Sparks. And here's the thing: healing 3 people taking 6K/s is not really harder than healing 1 person taking 6K/s.

If everyone remains stacked up and the portal tank is properly set up (a portal tank should have a resto back bar with combat prayer), they can heal whoever is in that AoE, and if they're spamming combat prayer to heal that damage from the AoE, then it makes no difference if it's one person standing in the AoE or three people standing in the AoE.

Now, in non-HM, you can get away with the portal tank not having a resto staff. 6K/s is spicy, but it's healable with self-healing and/or stacking yellow vigor. In HM, the portal tank needs to be responsible for healing the other portal players since the damage is doubled and can no longer be handled by just vigor and self-heals.

So, the short of it is that people are instructed to stack directly behind the boss to avoid Sparks, and the AoE is handled by healing through that damage with AoE heals. The upside of stacking is that it keeps people clear of Sparks. The downside of stacking is that if there are no heals, then only the person who has the AoE will die if people are unstacked, whereas all three will die if they are stacked--i.e., if you mess up, the consequences are worse. So just don't mess up.