Before reinstalling be aware that some ESXi version can't be installed on some old servers (you should check the hardware compatibility matrix VMware Compatibility Guide - System Search ) and check if your vcenter can list and add ESXi from different version (i.e. maybe a vCenter 6.7 can't list an ESXi 3.5... i don't know)
In general the update include more than just a patch, they contains many patches, drivers, extension...
The patch for Spectre/Meltdwon for ESXi 6.7 is known as "ESX670-201808401-BG" and seems to be embedded on the full update. VMware Knowledge Base
If you can, i advise you to update your servers to the ESXi 6.7 Update 2,, here's the release note for informations : VMware ESXi 6.7 Update 2 Release Notes