My assumption that it's most likely due to the jewel that he has on him. I'm probably overthinking this but if I were undine I would trust him in order to gain the jewel and if he does something funny boom the jewel helps me recover to potentially attack him.
I actually think it’s because she saw how the other one pointed his sword at the fl. However, she noticed the red one have been good to her so far and then like you said he got the jewel.
Probably because Vlemir is willing to go against the emperor for her, while Rohan cannot go against the emperor no matter what. The jewel is probably not the main reason for Undine to trust Vlemir over everyone at that place. Because it's hard to believe that not one villager would not posses 1 single pearl or any cheaper gems.
I’m on team Vlemir, but I don’t get why Undine chose him here