Good question since the Sil only says the "host of the Valar."  I never took it that the Valar themselves participated.  The army was the greatest that had fought against Morgoth, but it was momentarily set back by the winged dragons and Earendil had to slay Ancalagon.  If the Valar had been there, I would expect they would have been named and would most likely have been able to take care of the remaining dragons and balrogs in short order, and they would have been in charge of capturing Morgoth in the end.  Also, by that point in the First Age Morgoth had lost a considerable amount of his former power through spreading his stain all over ME.  Earlier it required a Vala like Tulkas to bring him down, but by War of Wrath his defeat could be accomplished with an army of Maiar, Vanyar, Noldor, and other lesser folk.  Does someone else know if one of Tolkien's other writings goes into more detail on the army?

I too have wondered why there weren't stronger guards around the Simlarils.  Eonwe may have underestimated Feanor's remaining boys a bit!

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"The world is changing: I feel it in the water, I feel it in the earth, I smell it in the air"
              --Treebeard