At one point I flew Snap Wexley (PS 6) with Veteran, making him PS 8 and then equipping Black One (which requires PS 7). At the time of the tournament, it was ruled allowed because you equip VI first but I have no idea if this was based on anything.

If this is allowed, would you. therefore, be able to equip Pulse Ray Shield on a Vaksai then equip Shield Upgrade?

Why or why not? What rule either forbids or allows this?

Yes.

Shield Upgrade increases your shield value by one, so you are now allowed to equip the Ray Shield.

Same thing with VI and Black One. VI increases your PS, so you may equip Black One.

Well to try and give you an answer I went to look at things like Autothrusters & Royal Guard TIE, other upgrades that require a condition to be met to use them, and sadly I don't have a 100% clear answer for you, only how I feel its supposed to work.

Autothrusters reads as "... You can equip this card only if you have the [boost] action icon."

Royal Guard TIE reads as "... You cannot equip this card if your pilot skill value is 4 or lower."

We already know that the Kihraxz is able to take an Autothruster if it first takes Engine Upgrade, and that TIE Interceptors (Saber Squadron Pilot in particular) seem to be able to take Royal Guard TIE if it takes VI as its EPT. This is similar to how Snap is indeed able to take Black One if he first takes VI.

Looking at Black One, it reads as "... You cannot equip this card if your pilot skill is 6 or lower."

As you already stated above, if Snap takes VI he is then able to take the Black One title. This interaction, along with the likes of AT and Royal Guard TIE, leads us to believe that the order in which upgrades are attached to a ship does indeed matter.

If we look at PRS, it reads as: "... You can equip this card only if your shield value is 1." And then if we look at Shield Upgrade, it reads as: "Increase your shield value by 1."

What I feel sets apart the order-of-equipping PRS > Shield Upgrade from all the other examples above is that in the 3 prior interactions, you're first equipping an upgrade that allows you to then equip a second one. In the case of PRS > Shield Upgrade, this is reversed. You're first equipping an upgrade that requires a certain parameter be met/maintained, and then violating it by trying to equip Shield Upgrade.

Just my 2 cents.

I don't think upgrade order matters, but rather the end resulting Ship you have built needs to be valid. So if after you are done equipping your ship do all the upgrades still check out valid? With the Pulsed Ray Shield and Shield Upgrade case, no they are conflicting. In the AutoThruster/Engine Upgrade and Black One/VI cases, after equipping all upgrades the ship is still valid.

This makes the most sense as an explanation as to why it wouldn't be allowed.

I'm pretty sure there's no such thing as "order of upgrades" essentially everything is considered equipped simultaneously. Snap can take black one because VI and BO come into effect simultaneously and BO can be legally equipped if VI is in play.

You can not give a Kihrax shield upgrade and PRS because SU invalidates PRS.