Catalysis of the reaction: a very-long-chain (3R)-3-hydroxyacyl-CoA + NADP+ = a very-long-chain 3-oxoacyl-CoA + H+ + NADPH. This reaction is the second (reduction) step of the four-step fatty acid elongation cycle in the endoplasmic reticulum that extends fatty acids of C-16 or longer with an additional 2-C unit.
very-long-chain 3-oxoacyl-CoA reductase activity