Now as most people will have already realised it is impossible to purchase everything from the vault (even just the characters songs and guitars for your achievements, while even 5 starring every song on all difficulties including all bonus songs. So here is what to do.
Simply play through on expert earning 5 stars on every song for $1800 (or which ever difficulty you can best manage). Then this allows you to purchase all characters, then simply complete hard for all guitars. Then simply delete your save file (having already got your achievements) and do the same again for all the bonus songs.