What is the expiation for not fulfilling a vow to not use contraceptives, and could the delayed marriage of the daughter be due to not fulfilling this vow?

You vowed not to use contraception, which is a permissible vow. In such a case, you have the choice between fulfilling it or offering an expiation for breaking an oath (kaffarat yamin). Since you did not fulfill your vow, you must offer the expiation for breaking an oath. There is no connection between your failure to fulfill the vow and the difficulty in your daughter's marriage. Your daughter is advised not to be excessive in her conditions for a husband and to focus on his religiosity and character.