Is what you said — a promise to Allah that you would be devout in your prayer if He healed you — considered a vow, and what are the consequences of being healed yet unable to achieve devotion in all your prayers? What is its expiation if it is a vow?

A vow to achieve khushu' (devotional humility) in prayer, conditional upon recovery from illness, is a vow that must be fulfilled. Khushu' is the submission of the heart, the stillness and presence of the limbs in prayer. If khushu' is present, no expiation is required. However, if forgetfulness and heedlessness overwhelm the worshipper, then an expiation for breaking an oath (kaffarat yamin) is required. Making vows is disliked and does not avert anything, but it is an oath that must be fulfilled if it pertains to an act of obedience.