Is there a reward for praying two supererogatory rak'ahs after every prayer to fulfill a vow, and is one sinful for abandoning them after fasting an expiation for not fulfilling the vow?

Undertaking a vow (nadhr) is disliked (makruh), and whoever vows an act of obedience to Allah must fulfill it. A conditional vow must be fulfilled if its condition is met. You must make up (qada') the two vowed rak'ahs for the entire period during which their prayer was interrupted due to an excuse. If the duration is not precisely known, continue making them up until you are reasonably certain that you have fulfilled your obligation. Fasting is not sufficient to make up for this vow.

Fulfilling a vow is obligatory and carries great reward. What you feel regarding the lack of reward is from the whispers of Satan. If you abandon prayer while being able to perform it, you are sinful, and fasting is not sufficient for you. If you are unable to perform them due to an excuse that is not expected to cease, then an expiation (kaffarah) similar to breaking an oath is sufficient for you: feeding ten poor people, or clothing them, or freeing a slave. If you are unable to do so, then fasting for three days.