There is no more vs seeker. Therefore don't be afraid of an N when you can reuse the most broken search attack in the game
This is only true depending on which turn of the game you're on. Nowadays, N's frequency is, on average, the lowest during the middle of the game when people are playing Skylas, Acerolas, Guzmas, and Sycamore for strategy to push their set ups forward instead.
VS Seeker is useless for early game, but great for mid to late game. Drawing VS Seeker is useless if you don't draw your supporters and use/discard them first.
Everyone now runs Tapu Lele GX at 3 if they can afford it, which is great for seeing supporters early game. 4 Ultra Ball, 3 Tapu Lele GX, 4 N in everyone's deck. That's 11 ways you can reach N at the start of any game. Early game N's are far more likely now that VS Seeker is gone and Tapu Lele counts and N counts are up to accomodate. Most people used to only run 3 N and 2 Tapu Lele GX because there was no point in cluttering your deck/risking an unoptimal opening Pokemon with excess counts when you had VS Seeker to recycle the first one you played.
When you're considering Alolan Vulpix vs Diancie vs Sylveon GX, you have to consider their primary role which is
getting set up, aka the first 2-3 turns of the game.
Your set up is fragile because of how much
early supporter access people have now. People literally go out of their way to Ultra Ball, Lele, N when I just use Beacon in Alolan Ninetales GX decks, and I'll tell you it works, and I lose games to getting N'd and not having my evolutions for another 2 to 3 turns while my opponent sets up since they play out as much of their hand as they can and then N me.
If I'm up against a Sylveon GX, I will do everything in my power to N away Magical Ribbon. You said it yourself. It literally is the most broken search attack in the game. That's exactly why I will always play N when I see it. I'm not letting anyone keep 3 cards of their choice.
You should always be afraid of N when using Magical Ribbon, and if people don't N your Ribbon that means they have confidence that their set up is better than yours even if you keep the Ribbon, or they bricked and they're going to lose the game to bricking regardless of what you do.
If you like Sylveon GX then go for it. All 3 set up options for Gardevoir GX have merrit. I'm not saying that Sylveon GX is worthless - I just don't like playing cards where you're not in control of the outcome. I also don't agree with your reasoning at all.
