Looking at it from 180-degrees, this could theoretically be less problematic for female FAs depending on how nitpicky you want to be. The loophole I see is that the male grooming standards are superficially less onerous but way more open to Vueling's interpretation. They don't define "clean and neat" in any way, so it's wholly open to interpretation by the airline or the purser or whomever is policing grooming standards. Haven't read the totality of the regulations but could they not simply deny a flight to a male FA who was subjectively "not" in compliance based on whatever the purser/grooming police thinks that means?
Compared to the male regulations, the regulations for female grooming is specific while levying more superficial requirements insomuch as foundation use, eyeliner, etc. But, using malicious compliance, female FAs could absolutely follow those rules but to disastrous results if they so choose. Just because you have makeup on, per regulation, doesn't mean it has to look good. Or simply do the bare minimum and force Vueling to prove you're in non-compliance. And without wording as nebulous as the male grooming standard, this could turn the tables. It'd be a lot of work, but ultimately it might actually be easier for female FAs.
Ultimately I applaud Veuling for at least taking the initiative to ensure their staff present themselves well (something the US3 need to learn about) - though unsure if this is the best way to do it since it enforces highly subjective rules as to "beauty", though indirectly. Their hard product and business ops, on the other hand, could use a TON of work though...especially as a resident of Catalunya that is sometimes begrudgingly a passenger of Vueling because of their route network in the Med/Adriatic. Case in point: a 757 (or A321...been a while) leased from who-knows-where in which two of the forward lavatories were missing toilet seats. Not just the lids, but the actual lid/seat combination leaving a literal hole in the commode base. FAs shrugged and responded with "Yo no sé." But at least they looked great while doing so.