Oh, right, duh.
I was very bothered when I saw it by this exchange between Ortegas and Uhura when they and Chapel are considering their options before entering the portal again.
"Should we even try to contact Enterprise about this?"
"I can't reach anything outside a light year's distance with all this interference."
So on the one hand, while watching it, I was very aware of the fact that they were still in the same star system when the dialogue forgot, but a few weeks later and I've forgotten myself. Scrying the wiki viscera now, it actually seems plausible, though, because the planet Kerkhov that has the moon that has the portal people is supposed to orbit another star that is itself in a binary pair, which seem to be collectively in a further binary orbit with the Vulcan sun. That's a plausible thing that does happen and could give you a light year of distance.
I thiiiiink it actually checks out though. I don't think the portal's been running the whole time since the portal people ascended to a higher bureaucracy. During the flight to the moon, Spock starts by explaining how they used to be an advanced civilization and then vanished. Well, how could the Vulcans even find that out, without studying one or more of the sites already, after the Vulcans had themselves developed interplanetary travel and possibly FTL to get over there? And then he says that he hopes the recently detected energy signature will answer the questions about the portal people, and Chapel asks whether that's the signature they have on sensors now.
The episode's opening logfile from Nurse Chapel mentioned that Enterprise detected the energy anomaly on the way to the Vulcan system, but also says that the reason they're going there in the first place is to investigate the moon. Isn't it the most plausible reason for Enterprise to be investigating in the first place, that scientists on Vulcan detected the giant flare that can be seen from well outside the system and tipped them off?
And if the Vulcans know of the portal people based on artifacts the left behind, I'm going to bet they've been to all the big sites, whatever those are. They've probably been to this same spot before and measured all those little pyramids, but if the portal just wasn't on at the time, maybe they also just couldn't do much to reverse engineer anything or find any handy encyclopedias?