The best way is to get your vanguard rank to 2 then you can buy two pieces of armour using vanguard marks that you receive from doing strikes and bounties. Then each time you rank up the vanguard you will get a package at the postmaster which can contain a legendary item and it will also give you a commendation which you can use to purchase more legendary armour.

In order to level up past 27 you have to get raid armor. Meaning you have to do raids like the vault of glass