IMHO it would be simpler to analyze the VHDL code rather than schematics or RTL, especially if the design is really large as you said.It's like as if you'd want to convert C code into assembler to understand it. Even if you don't know VHDL (or C), I think it'd be easier and faster to learn the basics of the high level language rather than decoding the compiled version (this is what I did as well when I had been in a similar situation).
The RTL viewer is a good option to view a large design that has not too many components in a hierarchy level. A block diagram gives understanding of the way the components are connected. The RTL viewer does not 'just' show RTL logic. To amalyse behavior I agree reading the VHDL will be easier (most of the time).