Do we need a Vertex Accelerator/Oracle Accelerator ?

Summary: Do we need a Vertex Accelerator/Oracle Accelerator to calculate taxes on Fusion when it has to get the taxes from Vertex( Third Party Tax system) . Could this not be achieved by a custom API ??

We are in the process of migrating from EBS R12 to cloud Fusion and we are trying to understand if the we can build a custom API to calculate the taxes on our OM and AR as we have a different pricing calculations needs . We are being told that fusion needs to be connected to accelerator to communicate to Vertex ? We are trying to understand if that is a mandatory or can be achieved though the custom API. If any one can share the knowledge.