Hello,
if you're planning to move your workloads from VLAN to NSX logical switch (LS) so sure you will use LS.
LS is a virtual subnet under NSX, this subnet will not be created on your legacy network. the gateway (SVI) for this subnet will be on an NSX logical router.
The port group (PG) is VLAN tagged, so it is a translation for the normal VLAN that exists on the physical network to be used under the vSphere environment.
So the port group and NSX LS could be attached to a VM, the difference that LS is under NSX everything logical and managed by NSX components and use the VXLAN concept but PG forward the traffic to the physical network to use the legacy VLAN concept.