The rating you gave for the Vexta is unipolar. Look here:
http://www.orientalmotor.co.jp/cgi-b...frameSize=56.4
The Vexta is rated 260 oz-in bipolar. But it is 2.8a, 3.2V bipolar parallel. I'd think about using a Xylotex to run it at 2.5a bipolar parallel @ 24V.
Or better yet, if you want to build your own driver anyway, look at the drivers at
http://www.embeddedtronics.com
These can be run at a higher voltage. High speed needs higher voltages. So, at a given voltage, the C or D windings should have a higher top speed because they have lower voltage ratings.