Yes, you can use Fiber Optic cables.  In order for that to work, you'll need Intel compatible SFPs for the appliance (qty 8) and the proper SFPs for the 10GigE switch side of the house.  Neither of those items are included with the appliance and will need to be added/ordered.  Also, for the TwinAX cables, make sure you get the ones compatible with your switch (i.e., Cisco cables won't work with Brocade switches). 
Hi, FAQ document says: Q: How is the VxRail Appliance connected to the customer network?  A: Each node of the VxRail Appliance is connected to a customer-supplied Top-of-Rack (TOR) switch, by either SPF+ (Twinax DAC cables) or RJ45 ports (CAT 6 cables) for cluster interconnect.  I would to like ask some questions regarding to above information Do we only provide 10 Gb Ethernet connection over DAC cables? or we have a Fiber Optic Cable option, too?  How can we be sure about which cables are compatible with our 10 GbE adapter options?   Best Regards Berat 
10 GbE Connection