Current version
The buttons say you need to talk to her about credits, but the buttons for items are still greyed when you have Savicite.
Try talking to Ula about credits first (Explain: Credits), then wait 24-hours after that. That should unlock the ability to trade
Savicite with the associated vendors, I think. I'll clarify the tooltip for the next version to prevent confusion.
Edit: Actually, correction, talking to Ula should only unlock being able to purchase using credits for select vendors (Nenne and Heidrun) while other vendors will still only take Savicite
or credits with a prompt (Kiona and Tuuva). The credits vendors
should have an option to use Savicite, especially if your character doesn't have enough credits. Figuring out the exchange rate depending on the items being sold might be a bit complex, but I will discuss with the team to see if that needs any change.