If an elder resigns, can the council appoint his replacement or does the congregation need to vote?

In the CRC church order (Article 4) it states that:

"In calling and electing to an office, the council shall ordinarily present
to the congregation a nomination of at least twice the number to be
elected. When the council submits a nomination which totals less than
twice the number to be elected, it shall give reasons for doing so."

The language of the above article is very vague and therefore almost anything could be justified under the clause, "it shall give reasons for doing so."  But traditionally, and the church order shows this through the supplement 4-a and subsection 4-c, the congregation is to have a voting voice in the selection of elders and other offices.

I can, however, tell you that his is not uniform throughout the denomination.  There are churches that do not abide in the spirit of this Article, but have taken liberties that could be justified under the wording of the Article.

If I recall correctly when this has happened in our church, the council nominated an elder who had just recently finished their term. They brought that single nominee to the congregation for a vote of affirmation (done in a very brief congregational meeting after a service).

They were then installed in a later worship service (again, probably an abbreviated form; similar to how we might if an elder/deacon wasn't able to make the main installation service).