"V" bar dips and Decline presses: trying to keep this short.
A) Dips, along with chins, are sometimes referred to as a body's motion through space exercise. As opposed to most any other exercise, where the body is somewhat stable and in a fixed position (feet planted as in squats & DL's, reclining/sitting on a bench, etc). With body through space exercises, they tend to call into action more joint/muscle response. Making the "V" dip a true compound exercise (one of the reasons chins and dips should be included in everyone back/chest program from time to time). Decline presses/fly's are limited in this regard.
B) Decline presses are an out in front and away from the body movement. "V" dip's (or any other dipping action) is an down, along the body plain, push movement. Different responses to the joint/muscle from either exercise.
C) Gironda used the compound "V" dips (and their different versions) to focus on that all important pec/delt tie in. The main reason he favored that exercise. If done correctly, it will also hit the inner and upper portions of the chest. (the whole pec structure is involved in most any chest pressing exercise, but there can be more focus on certain parts depending on hand position, angle and preforming of the exercise).
One thing that Gironda did not like were heavy developed lower pec's (along with big butt's on men...hence the Sissy squat). One of the reasons he had designed a better dipping exercise plan (over the regular dipping bar) was to direct resistance onto the middle/upper part of the pec's. The Decline press will target the lower pec's so much more, as will regular dipping exercise.
Nothing wrong with decline presses, very rewarding for a lot of men/women. But they are not to be confused with the superior, overall development that may be achieved with the "V" dipping bar. The decline press is not similar or just as good as the "V" bar.They offer different muscle responces. Good Luck.