Devoted Druid
Vizier of Remedies
Melira, Sylvok Outcast

Say I control these 3 pieces. Can I run the infinite mana combo? Melira causes the cost of Devoted Druid's ability to be unpayable but Vizier of Remedies's replacement effect changes the cost so it could possibly be paid.

So the question is, do I take into account replacement effects when deciding if a cost is payable or not?

Thank you.

You cannot activate Devoted Druid's ability in this situation. Placing a -1/-1 counter on Devoted Druid is impossible, and you cannot choose to perform an impossible action, so you cannot pay the cost of the ability. This is true even if you know a replacement would kick in that would cause something else to happen instead.