If the Vice President in the form of an obviously unjust attempt to remove a fully capable President via the 25th Amendment were to become acting President would they be eligible for impeachment as the Constitution allows a President to be impeached?
Impeachment is not unique to the presidency. Any civil officer of the US Federal government can be impeached and removed from office through the constitutional procedure. Most impeachments in US history have been of Federal judges. A Vice President could be impeached. So could an Acting President, whatever office such a person had previously held.
Also the 25th Amendment (section 4) process does not "remove" a President. It merely substitutes an Acting President until whatever disability or inability to perform the duties of the office the VP and Cabinet have certified no longer exist.