Start by creating a new Revision scheme (Tools tab > Administration > Vault Settings > Behaviors tab > Revisions... > New...). There, click on Scheme Format > New..., and make a scheme that has only two letters as Values: A and C. This can be picked then for your Primary Scheme Format (the first letter) used in the new revision scheme we are building.

Now, use "Numeric" for Secondary and Tertiary (not going to be used), and hit OK.

At Vault Settings > Lifecycles..., select the Flexible cycle and hit "Copy..." to create your own lifecycle, then make sure to create states (plus sign on the left panel) as needed. Here you will need two "loops" which are basically WIP -> Internal Release -> For External Review -> Construction/Issued Release, with transitions (left side of that window) build to go back from each of those back to the previous state. This allows you to have an internal release mini-cycle AND an external release master cycle above it.
At the same screen, select the "Internal Release" state and go to Transitions to select those coming OUT of the Internal Release and going down to WIP (or other below). Click Edit for those an change the "Actions" (tab) to select "Bump SECONDARY revision". That makes the drawing move from A.1 to A.2, A.3, etc. every time it goes to an internal release but comes back to WIP for edits.
Now select the transition from Internal Release state to another above (For External Review, or "Construction/Issued Release" state, if the path you have is straight that way) and Edit > Actions > select "Bump PRIMARY revision". What that does is change the revision scheme from A to C on the major letter once things bubble up to the upper cycle going for construction (no longer an internal drawing).
Finally, fix the transition from Construction/Issued Release state back to "For External Review" to include an action of bumping the SECONDARY revision scheme number. That will make your drawing bump from C.1 to C.2, C.3, etc.

Other tweaks you will need are just setting up your Category to assign your new Lifecycle AND Revision scheme as default. Vault Settings > Behaviors > Categories > Copy or make a new category then under "Lifecycles" at the bottom, assign your new lifecycle, and do the same for the revision scheme.
The only issue with this approach is that Vault will set the first version and revision of a file as purely "A", until you switch from internal release back to WIP to make it A.1, A.2, etc. But the cool thing is that Vault will respect the transition from Released -> For External Review and change the A.X number to a "C".
Related idea you may wish to vote for:
https://forums.autodesk.com/t5/vault-ideas/common-revision-scheme-requirement/idi-p/8000042