Yes, the illusory you has more mobility
Absolutely! The only requirements for Invoke Duplicity are (PHB, 63):
The illusion appears in an unoccupied space that you can see within 30 feet of you. As a bonus action on your turn, you can move the illusion up to 30 feet to a space you can see, but it must remain within 120 feet of you.
There are no restrictions except for total move of 30' per bonus action and a limit of 120' from you. It is simply an illusion and doesn't obey the laws of physics - just the laws of the Channel Divinity.
Vow of Enmity does not require concentration
Nowhere in the description of a paladin's Channel Divinity (PHB, p. 85) or the Vow of Enmity option (PHB, p. 88) does it mention that it requires concentration, so it doesn't. Using the ability simply activates it, and it does not require concentration to maintain.
Concentration is a rule that usually applies to spells, as detailed on page 203 of the PHB under it's own section. Other features that "hijack" the concentration rule make reference to spells, using wording such as "as if you were concentrating on a spell"1. In other words, if a feature is meant to include concentration, it explicitly says so.
RAW, using this option would not have affected your concentration on the hunter's mark spell.
1 As an example of this wording, stealing from this related question, an example of a cleric's Channel Divinity option, specifically the Trickery Domain cleric's Invoke Duplicity ability:
As an action, you create a perfect illusion of yourself that lasts for 1 minute, or until you lose your concentration (as if you were concentrating on a spell).
Hence this example would interfere with concentrating on a spell, but only because it explicitly says that it uses the concentration rule.
Best Answer
It's paladin levels
For these reasons:
The rules are written assuming only single class characters except when explicitly dealing with multiclassing.
There are vanishingly few features that use character level rather than class level. Off the cuff, Cantrip scaling and proficiency bonus are the only ones I can think of. It is explicit that they use character level.