The simple answer is that, in German, vieles is singular. This can be seen (although perhaps not absolutely clearly) by the fact that it ends in the neuter singular ending: -es.
That might sound like a bit of a cop-out, but natural languages are organic, and don't tend to be perfectly logical.
But just to help clear things up a bit more, vieles is one of a group of similar words, such as alles, nichts, einiges and manches, all of which are singular and neuter by default (although they can be declined in the plural in specific cases, as explained in @Cacambo's answer).
In a way, vieles can be translated both as much or many, as can be seen here, but is singular in both cases:
Vieles ist noch unklar
Much is still unclear
Er hat mir vieles erzählt
He told me many things