Praise be to Allah.
Before answering your questions, we would like to explain to you that making vows (nadhr) in the first place is makrooh, because of the report narrated by al-Bukhaari (6608) and Muslim (1639) from Ibn ‘Umar (may Allah be pleased with him), who said: The Prophet (blessings and peace of Allah be upon him) told us not to make vows, and said: “They do not avert anything, all they do is get something out of a stingy person.”
Ibn Qudaamah (may Allah have mercy on him) said: Making vows is not recommended, because Ibn ‘Umar (may Allah be pleased with him) narrated from the Prophet (blessings and peace of Allah be upon him) that he told the people not to make vows, and that he said: “They do not bring anything good; all they do is get something out of a stingy person.” Agreed upon.
This instruction not to do it is in the sense that it is disliked, not that it is prohibited (haraam), because if it were haraam, He would not have praised those who fulfil their vows, because the sin incurred by committing something haraam is greater than the act of worship they did in fulfilment of the vow. Moreover, if making vows was recommended (mustahabb), the Prophet (blessings and peace of Allah be upon him) and the best among his companions would have done that.
End quote from al-Mughni (10/68).
It is not obligatory to fulfil a conditional vow, unless the condition stipulated comes to pass.
Al-Kaasaani (may Allah have mercy on him) said: If it – i.e., the vow – is conditional upon something, such as if a person says: If Allah heals my sick loved one, or if so and so who is away returns, then I vow to Allah to fast one month, or to pray two rak‘ahs, or to give some money in charity, and so on, then the time [for fulfilling the vow] is the time when that condition is fulfilled. So long as the condition is not fulfilled, it is not obligatory, according to scholarly consensus.
End quote from Badaa’i‘ as-Sanaa’i‘ (5/94).
But if a person wants to fulfil his vow before the thing on which he made the vow conditional comes to pass, that is permissible, by analogy with offering expiation for breaking an oath (kafaarat yameen) before breaking it.
Al-Bahooti (may Allah have mercy on him) said: It is permissible to do the action that one vowed to do before the condition for it is fulfilled, like offering expiation after making an oath and before breaking it.
End quote from Kashshaaf al-Qinaa‘ (6/278).
For more information, please see the answer to question no. 102991.
The kind of difficulty which means that the obligation to fulfil a vow is waived is the kind of difficulty which makes a person unable to do what he vowed to do. So if a Muslim vows to do something that is beyond him, or he vows to do something that is within his capability, but then he finds himself completely unable to fulfil it, then in this case the obligation to fulfil that vow is waived, but he must offer expiation for breaking an oath, because of the report narrated by Abu Dawood (3322) from Ibn ‘Abbaas (may Allah be pleased with him), that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever makes a vow that he is unable to fulfil, the expiation for that is the expiation for breaking an oath (kafaarat yameen).”
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (10/72): Whoever makes a vow that he is unable to fulfil, or he was able to do it, but then became unable to do it, must offer the expiation for breaking an oath. End quote.
As for difficulty in general terms, which is something expected when carrying out any religious duties, and is also something normal in the case of a vow, which is an extra commitment in addition to religious duties, that is to be expected. In most cases the one who makes a vow wants to do something significant thereby, and he is showing that he holds the act of worship he vows to do in high esteem, therefore he has no excuse, and the vow is not waived in this case, so long as it does not turn out to be too difficult for him to do it.
If a person fulfils a vow to do an act of worship, then he will be rewarded for doing the act of worship that he vowed to do, and will also be rewarded – if Allah wills – for fulfilling that vow, because Allah, may He be glorified and exalted, praised those who fulfil vows, as He, may He be exalted, says (interpretation of the meaning): “They [are those who] fulfill [their] vows and fear a Day whose evil will be widespread” [al-Insaan 76:7]. Such praise can only be for doing something that is recommended or obligatory, which are both cases in which the doer will be rewarded.
Based on that, one who recites tasbeeh one thousand times in fulfilment of his vow will be rewarded for that tasbeeh, which is basically an act of obedience and worship, and he will also be rewarded for complying with the divine command to fulfil vows.
And Allah knows best.