As it says in the title. Can I become A Vampire Lord after siding with The Dawnguard? Ive already completed the quest "Chasing Echoes".
Yes, it is possible to become a Vampire Lord after having completed the quest "Chasing Echoes". Serana offers it to you once during that quest, and again after the quest "Kindred Judgement". Whether or not you have sided with the Dawnguard up to that point should not matter, but becoming a Vampire Lord will likely cause them to become hostile to you, which will cause them to attack you if you try and return to Fort Dawguard or interact with them. From the Wiki:
Harkon or Serana can grant this power during the main questline for Dawnguard. Harkon offers this power during Bloodline, Serana offering it twice, once during Chasing Echoes, and once more upon completing Kindred Judgment.
However, it is also possible for Serana to confer the power of Vampire Lord on you if she is currently your follower, and you have completed all of the main quests for the Dawguard DLC. Unfortunately, the wiki is not very clear about this. From the Wiki:
She can also be asked to bite the Dragonborn, which grants the ability to become a Vampire Lord, however this cannot be done if she has been cured of vampirism.