I didn't find a precise rule either, this answer is then partially guesswork. However, considering that, according to the rule below :
I would expect that Vandal Ouphe would see the ability as coming from Mayael who is now an artifact. Somehow, I feel this is the most natural way of understanding the rule. If they wanted the source to be the card in it's original state, they would have said it clear and the bolded line concerning the last known state of the card would not have been needed.
About your example, I am not 100% sure you can cast a spell after Liquimetal Coating has resolved but before Mayael's ability. This, however, does not change the answer of the question you're asking. (Edit : apparently you can.)
608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that's no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. The spell or ability is countered if all its targets, for every instance of the word "target," are now illegal. If the spell or ability is not countered, it will resolve normally. However, if any of its targets are illegal, the part of the spell or ability's effect for which it is an illegal target can't perform any actions on that target, make another object or player perform any actions on that target, or make that target perform any actions. If the spell or ability creates a continuous effect that affects game rules (see rule 613.10), that effect doesn't apply to illegal targets. The effect may still determine information about illegal targets, though, and other parts of the effect for which those targets are not illegal may still affect them.