Your Crucible and Vanguard marks converge into one and create Legendary marks upon the release of TTK, so keep your marks and you'll be able to buy new armour and weapons on September 15th. Granted, you probably won't be able to use them until level 40 but we'll have xp based levels so that won't be such a problem. Enjoy your new items! :) *EDIT* I seem to have given out misleading information. Thank you to Omega Prime1000 for the specific information. Both Vanguard and Crucible marks will be converted into commendations on a ratio to 50 marks to 1 commendation. Still, my initial advice stands, save those marks!