After researching this some more, it will take alittle more on my part first to get it working. It appears that I have to first call:

Retrieves a list of repositories from the server.

[Visual Basic]Public Sub ListRepositories( _ ByRef reps As VaultRepositoryInfo() _)
[C#]
public void ListRepositories(
ref VaultRepositoryInfo[] reps
);
Parameters
reps
The new VaultRepositoryInfo[] will be created and stored at the variable referenced by this parameter.

from:

Encapsulates one Vault server connection and all the source-control-related high-level operations a Vault client would need to perform. No Windows interfaces for gathering data are provided; all methods require full qualification. Look at the VaultClientPresentationLib for a higher-level interface that provides Windows UI where needed.

For a list of all members of this type, see ClientInstance Members.

System.Object
ClientInstance

[Visual Basic]
Public Class ClientInstance
[C#]
public class ClientInstance
Requirements
Namespace: VaultClientOperationsLib

Assembly: VaultClientOperationsLib (in VaultClientOperationsLib.dll)

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Has anyone already called Vault's ClientInstance.TreeCache from within VFP? The Tree Cache is a tree type structure full of files/folders. The files/folder objects will have object IDs, versions and ObjVerIDs which should help me locate the correct file to open for diff purposes.

TreeCache manages the local cache of the repository structure we fetched from the server.

For a list of all members of this type, see TreeCache Members.

System.Object
TreeCache

[Visual Basic]
Public Class TreeCache
Implements IDisposable
[C#]
public class TreeCache : IDisposable
Requirements

Namespace: VaultClientOperationsLib

Assembly: VaultClientOperationsLib (in VaultClientOperationsLib.dll)

It has properties such as CheckOuts, Load, Respository:

For example, the repository is called:
[Visual Basic]
Public ReadOnly Property Repository As VaultClientRepository
[C#]
public VaultClientRepository Repository {get;}