I recorded a set of vibes a few years back. I originally wanted to do the Marimba but it was a huge one so I opted for some smaller vibes.
We set up two Beta SM58's over the top of the keys. They were about 3-4" above the top of the flat/sharpe keys. Came out nice but not exactly how I wanted them to sound. I think we should have experemented with the mic models/brands more.
I wish I could be more help. I so love the sound of the Marimba. At least it is something to start on. Maybe try SM57's on them. I think the top is the best placement rather than the bottom of the tubes area.
Good luck!
jontflesh