I would say that the design here is not different than other stretch clusters, you need to have at least one Veeam repository per site, and be sure that proxies are mapped to local repos using as Anton suggested the proxy affinity. Still, with VSAN you may want to have in an ideal design one virtual proxy for each ESXi node, and disable drs for them so that they don't fail over to other nodes, or even worse to the other site. If a VM moves to the other site, it's picked by the proxies at the other site.
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1