There's been some recent discussion of rolling all the dice out in the open. It's encouraged by the designers, but apparently some people still feel the need to roll at least some of the dice behind a screen. This has been a tradition since D&D1, but is completely unnecessary in this game. However, to roll everything openly takes a bit of adjustment, some of which is outlined here:
https://community.fantasyflightgames.com/topic/197716-i-roll-all-dice/?p=1974773
I thought it might be useful to flog this horse a bit further with an example...the horse may be dead soon, but it might have a bit of life left...
So in the post above, one change is making sure you only ask for rolls when you need to. You can assume the NPC is stealthy until you want the players to have a chance to inject narrative elements. When you ask for a roll, it's because you have something in mind for both success and failure.
Another change is that the player usually rolls when they are involved...they get the positive dice, opposed by the NPC's negative dice. However, this still works if you roll for the NPC, giving them the positive dice opposed by the PC's negative dice. There are some Talents the PCs can take that can affect NPC rolls against them. It's only fair to give them the option. You can flip the benefit (e.g.: give boosts where a Talent removes setbacks) but it's not as precise.
This is how I'd narrate the results of a PC's Vigilance vs Stealth (flip it if you're rolling openly for the NPC):
Failure + 3 Threat: "Something is not right, and it's creeping you out, your hair is standing all on end. You take a Strain, and you have a setback on your next action."
- it doesn't matter what the NPC dice pool is for this result, the PC is at a complete disadvantage, and the NPC can capitalize on the setback.
Failure + 2 Advantage: "Something is not right. You look back and briefly catch a face looking in your direction, then a tall Ithorian walks in front of you and the face is gone. It might be nothing, but if you saw them again you might recognize them."
- I might change this up depending on the opposing dice:
If the NPC dice pool is low I might say "You're sure someone is following you. They don't seem very good at it, but a tall Ithorian walks in front of you and the person seems to have ducked back into the crowd. You can't see them now."
If the NPC dice pool is high I might say "You're sure someone is following you and briefly catch a face looking in your direction. You can't tell if they've seen you notice them, but then the crowd swirls and they're gone. It might be nothing."
Success + 3 Threat: "You spot someone looking in your direction, and it occurs to you you've seen that face recently. You lock eyes for a moment and you (recognize/remember/get a good look), but the crowd shifts and they're gone. Take a Strain, and you have a setback on your next action."
Success + 2 Advantage: "You're being followed, you know this. Through the reflection in the shop window you can see them keeping tabs on you and your party. They don't seem to have noticed that you've spotted them. You can take a boost on your next action, or maybe you have another idea..."
- Again, I might change this up depending on the opposing dice. Narrating the NPC's apparent skill can take the impact (if any) out of the reveal.
Just some thoughts...