Hi,
What I get is you trying to ping from the PE to remote branch and this PE has more than one interface under the VRF. you are asking about which interface will the router will use as source interface on ping
From my experience it is using the lowest numerical value if you have 192.168.x.x, 172.16.x.x & 10.10.x.x it will use 10.10.x.x as a source address. you could check by your self by simulating this on small LAB and configure reverse route on the remote PE one by one and see the result.
Regards,
Mohamed Elhariry
JNCIE-M/T # 1059, CCNP & CCIP
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