I'd try a similar variation

Select b.cfk1, b.cfk2, a.ctype;
	FROM  vcf!many b ;
	INNER Join vcf!one a ON  b.cfk1 = a.cid;
        WHERE (b.namount1 + b.namount2 <> 0) or ;
              (b.namount3 <> 0 and a.ctype$"F8" ) ;
	GROUP By b.cfk1, b.cfk2

which should use short circuiting a bit better.
Perhaps an index on namount1 + namount2 in vcf!one
speeds things up - check the optimizing results,
since I am not sure about <> being optimized.
If it isn't, try

WHERE (b.namount1 + b.namount2 > 0) or ;
            (b.namount1 + b.namount2 < 0) or ;
            (b.namount3 <> 0 and a.ctype$"F8" ) ;

HTH

thomas