An additionally expensive way, if you already have the gear/weapons you want, is to use your marks to buy the cheapest gear (arms/feet) which are 65 marks, and break them down for 3 shards. And of course if you look up the time cycle for events you can actually do 3-4 an hour, and easily hit your 100 vanguard marks in no time as well as a 10-20% chance each time you complete one, getting another shard/energy. (and of course while waiting between events you can do extra material/rep farming... though this can get monotonous. Best way to do it if possible is with a fireteam, far more enjoyable and feels less grindy.