If the Substitute takes damage equal to or greater than 25% of the user's HP, it will disappear (the user's defensive stats are used in damage calculation). OHKO moves will always break a Substitute. A Substitute is capable of being Baton Passed. Does not prevent damage from pre-existing status conditions. Draining moves break the Substitute and restore HP. Recoil moves do not cause the user damage if used against a Substitute. If the user switches out, the Substitute is lost.
Knew it lol.
So this is why subs are so usefull on baton pass teams.
Not only can they be passed and used to take an extra hit but at +6 def Vaporeon is hard to beat.
Anyway I have highlighted the important part in bold text which highlights the fact damage calculation is the same as the users just with less hp.
So boosts effect your defs therefore boosting the subs.