Vector was within the Allspark/The Crown (the top of the tree, which we never actually reach in Ablyss) when he sensed Galvatron being pulled back to reality and so he jumped in to intervene and fight him
The Crown is possibly superior the Astral Plane or is relative to it at the very least, i lean toward it being relative as obviously thats where Primus' astral form manifested when he was fighting Unicron, before he trapped both of them in corporeal forms in the multiverse
idk about the Tree necessarily being Omniversal, personally i think it kind of depends on your definition of Omniversal
this is about to be a tangent but bigger than infinity cosmology stuff is super confusing and im pretty sure OG made a thread on it a while back
however, i will say, if you mean Omniversal as in just being somewhere beyond Megaversal...

i think the TF cosmology could get you there, possibly
so just one universe produces an uncountable infinity of branches, which iirc i asked OG and thatd be an extra cardinality up, so
universe+branches= multiverse+ - megaversal+
theres infinite of these universes in a cluster= megaversal++
theres like a dozen known clusters, and honestly theres probably clusters we havent seen but this is the low end, thats still multiple sets of infinite megaverses at
LEAST theres 17-22 higher dimensions each containing universes just like those beneath it
and then the Tree of Life is
so vastly beyond all of that that it isnt even
real by comparison, and thats all before you even
REACH The Crown/Allspark