you put a line in home/.config/openbox/autostart.sh like:

vlc&

to start vlc at startup, probably you can tell it to start minimized even in this line. but I don't know how :)

but you can edit rc.xml in the same folder. down is an "applications" part where you can tell to start certain applications minimized, undecorated, maximized, fullscreen etc..

just see how it's done there, it's really easy to figure out. only drawback: your program will then always start like this.