public BigInteger getBaseOffsetFromVbt() {
		return virtualBaseOffsetFromVBTable;
	}
Returns the virtual base offset from VB table.  This seems more like an "index" into
an integer array having values such as 1, 2, 3
@return the offset
Ghidra/Features/PDB/src/main/java/ghidra/app/util/bin/format/pdb2/pdbreader/type/AbstractVirtualBaseClassMsType.java