When I support I rush Sightstone.
Ardent is mid/late game item anyhow and not early game and if your team does not see the importance for vision then mute them since they have no common sense.
Whenever I have enough gold to upgrade my gold item I do it as early as possible I also have enough to buy a control ward and make sure my ally has backed or is not in danger before I leave. Most gold items upgrade into a vision item so generally go that route to allow more utility to fit on my kit, and upgrading gold item early provides more gold overtime making other items more affordable. Following that gold upgrade its one more back early game finishing the sightstone.
There are far better items to rush then Ardent that benefit you team and not just the ADC.
No point in being that guy, the squishie support who is always dead so never there to assist with Ardent passive in team fights. I would sooner rush redemption over Ardent after I build vision any day. Nights vow is also a good pick as well as locket. Adds strength and protection for me, and has its benefits for the team and my lane partner.
Personally, I would go gold item into vision, redemption, locket, ardent and then your pick depending on situation ie, building either mixed ap and defense or ad and defense or just stacking armor etc. I played a game a few weeks ago and literally had 5 items with actives as part of my kit for utility and they were all worth far more then ardent.

