She vowed to give one-quarter of her salary in charity – can she give it to her father who is poor?

If a Muslim wants to give voluntary charity, he should look
at the situation of the needy people around him and give precedence to those
who are in greatest need. It is better to give to relatives, as the
Messenger of Allaah (peace and blessings of Allaah be upon him) said: 
“Start with those for whom you are responsible: your mother, your father,
your sister, your brother, then the next closest and the next closest.”
Narrated by al-Nasaa’i (2485); classed as saheeh by al-Albaani in Saheeh
al-Nasaa’i. 

In Saheeh al-Bukhaari (1368) it is narrated that Abu
Talhah (may Allaah be pleased with him) said to the Messenger of Allaah
(peace and blessings of Allaah be upon him): The most beloved of my wealth
to me is the garden of Bayruha’, and I am giving it in charity to Allaah,
hoping to find reward for that with Allaah. So dispose of it, O Messenger of
Allaah, as Allaah shows you.’ The Messenger of Allaah (peace and blessings
of Allaah be upon him) said: “Bravo! That is a good deal, that is a good
deal. I have heard what you said and I think that you should give it to your
relatives.” Abu Talhah said, I will do that, O Messenger of Allaah. So Abu
Talhah shared it out among his relatives and cousins (sons of his paternal
uncles). 

Since you did not mention a specific person or cause when you
made this vow, there is no reason why you should not give this charity to
your father who is in need. Al-Bukhaari narrated in his Saheeh (1333)
that Abu Yazeed Ma’n ibn Yazeed al-Sulami said: My father Yazeed used to
give dinars in charity and leave them with a man in the mosque. I came and
took them, and brought them to him. He said: By Allaah, I did not mean them
to be for you. I referred the matter to the Messenger of Allaah (peace
and blessings of Allaah be upon him) and he said: “You will have what you
intended, O Yazeed, and to you belongs what you have taken, O Ma’n.” 

Al-Haafiz said in al-Fath: It is as if he did not
think that it was acceptable to give charity to his son, or that it was
better to give charity to strangers, but the Prophet (peace and
blessings of Allaah be upon him) explained to him that he would have what he
intended, because his intention was to give charity to one who was in need,
and his son needed it, so it went to the right place, even if it had never
crossed his mind that he would take it. And he explained to his son that he
had the right to keep what he had taken, because he had taken it in a lawful
manner and he needed it. End quote (2/292). 

It may be understood from this that it is permissible to give
to fathers who are in need voluntary charity or something that you have
vowed to give without specifying a recipient, and in fact they are more
deserving of it than others. There is nothing wrong with you buying a house
for your father with this money. 

And Allaah knows best.