Hello Jean,
You can achieve the same in many ways, but the easiest of them all would be the following:
Create two different portgroups with these two VLNAs (VLAN A and VLAN B) in the vSwitch/dvSwitch. Create External Networks in vCD using these two portgroups.
Now configure two Organization networks and connect these two Org networks to these two External Networks. This was whatever vApp's connect to these Org networks will essentially be connected to two different VLAN's (A & B).
Now create two different vAPP's with the respective VM's in then (vAPP A and vAPP B). Now you connect vAPP to both external Networks (VLAN A & B). But connect vAPP-B only to external network (VLAN-B)
For example,
Both the external networks to vAPP-A and vAPP-B.
Now put Processor, Exporter, DB/File Server in vAPP-A, but connect Processor, Exporter VM's only to external network A (VLAN-A) of vAPP-A whereas DB/File Server should be connected to both the external networks (A & B). In vAPP-B put all the converter VM's and connect them the external network B.
This way what you are trying to achieve can be done.
There are other ways as well, but this will be easiest.