General Notes: Usually it s best to wait for the highest rank of your corruption before buying. This is more efficient, often both in terms of Echoes to Corruption ratio AND in terms of saving slots on your character. If you have a pressing short term need (i.e. progression), then go for the cheaper version, sure. Ny alotha Weapon specific corruptions like Lash of the Void will almost definitely NOT be on the vendor, but are included here, italicized, for completeness.
Ineffable Truth is more efficient at value per corruption at Rank 1 than Rank 2 - 30% CDR for 12 Corruption is better than 50% CDR for 30 Corruption. Higher rank remains more slot efficient, however.
The corruption vendor cycles every 4 weeks.