I currently have two 2348TQ's connected in a vPC to two 7009's in a vPC running 7.2(2)D1(2) via four F312FQ-25's (two connections per 7009). I am running into an issue with some Dell 720's with Intel 520X NIC's connecting to the 2348's. We have installed the latest drivers for the NIC's on the servers. Once they are connected they will ping three times and drop off. If I wait about 30 minutes they will ping 3 times and drop off again. I have connected a laptop to the 2348's and verified pings to the laptop with no issues. The Dell servers have ESXI 6.7 installed.
This is looks more like Vmware vSwitch config or Dell network card driver issue as per your description.
check once again FEX port-negotiation settings. before you open a case with VMWARE or DELL.
This kind of issues usually happen due to a "received missed errors" after applying drivers or missing new drivers and I have seen many cases now in different ESXi versions. Below I will share a link where there is a couple of tests you could to verify what might be the problem:
You could use a command like the following with the vnic with the problems to verify the counters and discard whether or not it has something to do with it:
# esxcli network nic stats get -n vmnic2
Below I will also share a couple of bugs for servers that have this Intel NIC 10G, where they have found ways to disable and enable features to reproduce them and also waiting for a solution:
Keep me posted,
Please qualify the helpful answers!