ITT, Vanessa and her supports are overrated

First things, she barely gets much time to support Boulder in C2 since she's off rescuing Ross. C3 is another short chapter so again not a whole lot of time to work with, and the mov gap still has to be taken into account anyway. She barely gets any points with Lute in C4 [Well, in theory you COULD have her run off to the village, she obviously gets there the fastest] but if we do that to have Lute actually do something, no time for Boulder.

Then factor that most other earlygame chapters are short and the move gap is always a problem [Since Boulder and Lute are on the back lines that just makes things worse] and I can't see her supports activating in any reasonable timeframe, and she is completely reliant on them, plus I can't think of any scenarios in this game where flight utility is actually useful [Like Thany airlifting Dick/some other guys over the wall in C11A, honestly this is the only reason why I don't argue against Thany in UpMid anymore] and while I don't necessarily want her out of Top tier or even lower on the list[though a case could be made], I do think she's getting a bit of a free pass on her supports.

plus I can't think of any scenarios in this game where flight utility is actually useful

Really? You just mentioned some earlygame ones.

Midgame:

9 Eirika: fly over the ocean and visit villages before the pirates do (you can do this without getting attacked at all if you're smart)
10 Eirika: dump someone on the mountain stairs for low turning, or intercept peg reinforcements
12 Eirika: general flexibility

13 Ephraim: rivercrossing

15: loldesert
17: rivercrossing
18: get to Gorgon Eggs
20: massive amount of terrain
F: bypass the middle gap to skip the Draco Zombies

That's not all, but it's most of it.

Also, Vanessa doesn't require full supports the entire game. Stuff like a B and C is good enough.

Let's see... for a frame of reference, Vanessa with B/C Moulder and Lute gets 3 att, 3 def, 15 avo. Compared to, say Kyle, who gets soemthing like 2 att, 2 def, 17 avo from A Forde/B Eph. And that's full supports for Kyle. Or switch his B Eph to B Lute, his def bonuses now go to 3, it doesn't really matter. So you can see that if we were to compare Vanessa to Kyle, we can, in a worst case scenario for Vanessa, ignore them because the bonuses for both units will be roughly the same (in a best case scenario where Vanessa will have an edge in supports, those bonuses will be highly relevant, such as when Kyle immediately joins and Vanessa might have Cs, or gets Cs before Kyle does).

So anyway, onto a direct stat comparison. Before promotion, Vanessa wins offense. At level 5, it's 2-3 str and 5 con vs 6-7 spd, which means Vanessa can either use steel lance and close the att gap (and lose her spd lead) or stay faster by using iron or even slim or something. Of course neither will actually be at level 5 by the time they coexist, but... their differences in growths makes the raw differe (Kyle wins str by 15, Vanessa wins spd by 20, though she spd caps by 16/0).

Defensively, Kyle certainly wins. He has ~2 def and 6 HP (though the HP scales very very quickly due to winning HP growth 90 vs 50), swords/lances vs lances, and no weakness to bows. And can also utilize terrain for durability. Most of these are small but they add up. Vanessa only has a small res lead, and her avoid doesn't really matter since it's not reliable at this point.

But Vanessa has flying, which has plenty of uses (as Mekkah pointed out). YOu could argue the durability gap > the offense gap, but flying more than compensates.

After promotion, the spd lead is about 7, while Kyle increases the gap in raw stats elsewhere (3 def, 10 HP, 5 str, 8 con). However, it's certainly worth pointing out that Vanessa's avoid is reaching terribly reliable amounts (at 20/1 with a B/C anima, that's 72.5 avo. Kyle at 20/1 with A Forde/B Eph has 56). Even before promotion, at 16/0 with, say, double Cs, she'd have 61.5 avo, which isn't bad at all. What used to be an easy durability win for Kyle is not so clear cut anymore, and taht's before Vanessa's supports are even finished and Kyle's are.

Admittedly the offense gap is a lot more debatable now. But again, flying.

So Vanessa has fairly even combat with Kyle. Perhaps losing a little in fighting overall. However the flying utility makes me feel there is indeed a tier gap between the two, justifying Vanessa's position in top tier since Kyle is certainly at least high tier. I suppose her actual position within top tier can be disputed, but not her being in top tier.