What is the best thing a woman can vow to offer to Allah -the Exalted- if He blesses her with a righteous husband?

There is no need for excessive worry due to delayed marriage. Rather, one should rely on God and trust in His grace, for all of God's decrees are good for the believer, even if they appear otherwise, in accordance with the Prophet's (peace be be upon him) saying: "Wondrous is the affair of the believer, for all his affairs are good for him..."

As for the vow, this vow of yours falls under the category of an ambiguous vow, and it entails an expiation for breaking an oath if the condition upon which the vow was suspended is met. It should be noted that this type of conditional vow is disliked (makrooh), and some scholars have even deemed it forbidden (haram).