I've heard both DOO-vƏl...
That's pretty accurate, but if you want to be picky, the "OO" is a sound that isn't in the English language. The best way I can think to describe it is it's somewhere between "UH" and "OO" but focused just behind the teeth.
Like the french tu
Yeah, close enough anyway. The OO sound does exist, like in Hoegaarden. No, it doesn't sound like Santa Claus landing in a vegetable patch. While I'm at it, the guttural "g" that's used in English, French or German isn't used in Flemish. The Dutch pronounce it like they're trying to hack up a wad of phlegm, while in Flemish it sounds more like you're trying to expel a hairball.