Unless you or a previous owner has put dirt in the cooling system, your heater core should be just fine.
I bet the louvers are hanging shut inside the heater box. Or, the T-stat is not functioning properly .... thus, engine is running well below normal!
'84 Vanagon '02 ALH TDI engine(32 to 35+ MPGs consistently) Malone Stage 1 tune. Road trip to Nova Scotia 2013. Road trip to Alaska summer 2014 & 2016, 11k & 12k miles. 71k miles on install, Jetta Cluster odometer currently at 200kk miles.