The powers-to-be in the birding world recently split the Yellow Warbler into two species: Northern Yellow Warbler and Mangrove Yellow Warbler.
The Mangrove Yellow Warbler is a resident in Puerto Rico and the Virgin Islands. They are debating whether Northern Yellow Warblers occur as far east as Puerto Rico during migration and winter, from August to May.
Daphne Gemmill has found that the Mangrove Yellow Warbler on Vieques is not as distinctive as some of the other subspecies within Mangrove Yellow Warbler. She finds it very difficult to tell them from the Northern Yellow Warbler.
According to Birds of the World, aestiva migrates through the West Indies, while rubiginosa is considered a vagrant with one record from Cuba. The field marks for the Mangrove Warbler, Puerto Rican subspecies (Setophaga petechia cruciana), show that the male's crown is mottled pale rufous, and the streaking below is heavier, whilst the upperparts are marginally paler.
From eBird - Mangrove Yellow Warbler includes many subspecies, ranging from birds with bright rufous heads to birds with small rufous caps—and in a few cases, no rufous at all. Northern Yellow Warbler subspecies vary in brightness but consistently have a yellow head without any rufous on the cap, head, or face. Females and immature birds of both species are typically a duller yellow and can be hard to differentiate.
From Jeff Gerbracht at the Cornell Lab of Ornithology - One other difference to look for is that the wing projection is noticeably longer in the migratory Northern Yellow Warbler than in the resident Mangrove Yellow Warbler.
You may also find these links to be helpful!
https://birdsoftheworld.org/bow/species/manwar1/cur/identification#simspecies
https://birdsoftheworld.org/bow/species/yelwar1/cur/identification#simspecies
Northern Yellow Warblers are considered rare on Vieques, so all Yellow Warblers on Vieques are Mangrove Warblers. When in doubt, choose Northern/Mangrove Yellow Warbler in eBird.