Scotbill/Proplinerman...Well they seriously looked at the next move from turboprops (Unducted Fans UDFs) after the 1970s Fuel price inflation but UDFs had development problems and the very efficient CFM-56 arrived. (There was an Air&Space article on the Boeing 737-300 in which a Boeing engineer said they only expected to sell a few hundred before UDFs became the norm
)
Doesn't a high by-pass jet engine have a parts count advantage over a turbo-prop engine+propeller and less Mach number limitations too.
Another economic disadvantage of the Vanguard was having four engines rather than two (there wasn't a turboprop engine large enough to make it a twin)
It might be interesting to compare the costs of a 737-600 or A318 with the largest HS/BAE/'Avro' 146