(Seldan is going to harass some other Companions for more context on Karis in hopes that he'll have anything useful to say.)
...He agrees that Karis was not in a position where she should have been defying Vkandis before this. And it's not obvious that she should be now according to her values, he's not sure to what extent she endorsedly believes that she owes Vkandis favors that would be costly to her personally because He helped her end the war in Karse, which - it's sort of debatable how much He helped, it was at that point already determined that Valdemar was going to win, but it certainly made the aftermath cleaner and there's Good in that.
He expects she's torn because...well, Vkandis' actions would make a lot of sense if Leareth were like Tar-Baphon, right? A lot of people, including some of the Heralds, kind of believed he was like that! Blai himself thought that it could conceivably be worth taking much more destructive actions than Vkandis has so far if the alternative were Leareth making an evil god.
Leareth does not seem to be very much like Tar-Baphon, though. Seldan's read on him is that he seems to have genuinely wanted to make a Good god, or - maybe not exactly that, by Blai's world's standards, but the closest thing he could conceive of given the Velgarth baseline for what one can expect of gods. Even over a few brief conversations, he...reminds Seldan of someone, someone he's fairly sure he respected, though Seldan is completely failing to remember who because being reincarnated as a magic horse has some serious downsides. He seems pretty genuinely committed to avoiding war with Valdemar; if he weren't, he would surely have gotten out of the kyree caves and not ended up almost dying permanently in order to get Jisa out safely, which has to be a big deal, you don't get to be two thousand years old by taking risks like that casually.
Anyway. Karis knows that, and - she has a lot of trust in Vanyel, who's thought for a long time that Leareth might be someone they could find a way to work with rather than fight, and - she also knows that Vkandis, or at least His agents on Iftel, were willing to do all this and apparently willing to invade Valdemar over it.
So the question, maybe, is: how do you weigh it up, when your conscience says one thing, and your god, who presumably knows a lot more about the bigger picture, says another? And how to take into account that Vkandis is really not the same as Iomedae, and may not be as straightforwardly Evil as Asmodeus but does not, for example, seem to have considered trying to talk Leareth down from his course of action?
...He could probably come up with actual advice given long enough to mull on it, but he doesn't have any pre-prepared. He can recall maybe having written about similar dilemmas in one's service to a human king, but it's not actually the same.