Oliver Cowdery initially wrote “that it were in vain” in 𝓞; then virtually immediately he corrected the were to was, crossing out the were and supralinearly inserting the was (there is no change in the level of ink flow). Elsewhere the text has examples of only “it was (all) in vain”:
The critical text will maintain the singular was here in Alma 27:1.
Summary: Accept in Alma 27:1 the correction of were to was in 𝓞: “that it was in vain to seek their destruction”.