Hi Tobias,
FHRPs are designed to work with vPC and with HSRP can actually be active/active from a forwarding perspective. One switch in the vPC pair will still have the active HSRP role and it will be the switch that replies to ARPs for the HSRP VIP. Both switches in a vPC pair will forward traffic received with a destination MAC address of the HSRP vMAC.
The main restriction with vPC and L3 is that you cannot peer with a L3 device over a vPC. Since FHRP are layer 2 from the host then layer 3 to the next hop as long as your not using a dynamic routing protocol to a next hop over a vPC then you should be fine.
You may also look into leveraging vPC features like ARP sync for active/active HSRP to improve convergence time.
Starting on page 69 of the vPC best practices guide talks about L3 restrictions and active/active with HSRP. http://www.cisco.com/c/dam/en/us/td/docs/switches/datacenter/sw/design/vpc_design/vpc_best_practices_design_guide.pdf
I'm a little confused with your diagram on where your next layer 3 hop is. FabricPath between the data centers is L2. I see two firewalls in your diagram, are those your next L3 destination?
If so I would recommend plugging the firewalls into the N5Ks not the FEXs and then you could run your HSRP on one pair of your N5Ks.
You might also look at AnyCast HSRP since that will allow all 4 of your N5Ks to actively participate in HSRP and forward traffic. This will prevent you from crossing between DataCenters to get to the default gateway. AnyCast HSRP is supported on the 5500s no so that would be my recommendation. http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5500/sw/fabricpath/7x/fp_n5500_config/fp_n5500_interfaces.html#pgfId-1693449
