Finale 2012
VDL 2.5
Writescore template

How do I set it up to have cymbals on the same staff as mallets?  Much like in the marching setting where the mallet players will have cymbal rolls / hits during parts.

Thanks in advance!

You have 2 options.

The first is to use the suspended cymbals that are part of the marimba and vibe instruments. They are mapped above the high end of the instruments range. You'll want to apply a rhythmic notation staff style to those notes after entry.

The second option is to use an instrument change and switch to one of the rack combos which contains the sounds you wish to use. The steps for doing this depend on whether you use Finale 2012's Instrument Change, assign the cymbal sounds to a different layer, or use a bank of instruments containing the keyboard and cymbal sounds you wish to include.

Your specs say VDL 2.5. If you haven't already, you will need to update to VDL 2.5.5 to use the Cymbal Rack Combo. I'd also suggest reviewing the Finale tutorials and help guide for instrument changes and applying staff styles.