[You cannot request a veggie meal in advance since veggie meals are a standard option although they can run out in Y before the food trolley gets to you. Its one of the VS failings. You have to look around for a real special meal option like vegan to put in a meal request in Y.
You can request a Vegetarian meal in advance, even if you are travelling in PE, or any other cabin. While VS always carry Vegetarian options as part of their three main meal choices, they do recommend ordering a Vegetarian meal if you are flying in Economy just incase they run out. There are also varying degrees as to what constitues a Vegetarian meal.
If flying in PE, there will be no need to pre-order a standard Vegetarian meal as there will be enough onboard. If they run out of Vegetarian meals in PE they will take them from Y for PE pax.
Almond, you can have a look at the recent trip reports, as suggested, but there is no guarantee as to what is onboard until you get there. Even if a certain meal choice is planned, all sorts of things can happen to prevent that meal choice from being loaded, so i would make a choice between either pre-ordering a special meal as a fish option, or making do with the vegetarian option. Fingers crossed you may be pleasantly surprised to be offered a fish choice as standard.
One word of warning if you do pre-order a Vegetarian meal, is that there are two kinds - Vegetarian and Lacto/Ovo Vegetarian, the second meaning that Eggs & Milk are allowed. If you do not request the Lacto/Ovo, your meal will essentially be a Vegan style meal - trust me, i know from experience! The standard, on-board, non-pre-ordered meal is not this style, but Lacto/Ovo.
I hope this helps!
Thanks,
Sarah