She vowed to do Hajj but she has not done the obligatory Hajj

It should be noted that vows are not allowed. The Prophet
(peace and blessings of Allaah be upon him) forbade them when he said:
“They do not bring any good.” Hence some of the scholars are of the view
that vows are haraam. Why should you make a vow? Why should you burden
yourself? Will Allaah not heal you or your relative unless you stipulate
conditions for Him? Subhaan Allaah! Do not make vows, rather ask Allaah for
healing. If Allaah wants to heal him, He will heal him whether you make a
vow or not. If you do that and you make a vow, if it is a vow to do an act
of worship then you are obliged to fulfil it, because the Prophet
(peace and blessings of Allaah be upon him) said: “Whoever vows to obey
Allaah, let him obey Him.” 

Based on this, we say to the woman mentioned in the question:
You have to go for Hajj, but start with the obligatory Hajj, then do the
Hajj that you vowed to do, because it is obligatory for you, and if you do
not do it you are exposing yourself to a grave punishment which Allaah
mentioned in the verses where He says (interpretation of the meaning): 

“And of them are some who made a covenant with Allaah
(saying): ‘If He bestowed on us of His Bounty, we will verily, give Sadaqah
(Zakaah and voluntary charity in Allaah’s Cause) and will be certainly among
those who are righteous.’

76. Then when He gave them of His Bounty, they became
niggardly [refused to pay the Sadaqah (Zakaah or voluntary charity)], and
turned away, averse.

77. So He punished them by putting hypocrisy into their
hearts till the Day whereon they shall meet Him, because they broke that
(covenant with Allaah) which they had promised to Him and because they used
to tell lies”

[al-Tawbah 9:75-77].

They vowed that if Allaah made them rich, they would give in
charity and would be righteous. Allaah gave them that, but they were stingy
with the money and turned away from righteousness. “So He punished them
by putting hypocrisy into their hearts” – until when? “till the Day
whereon they shall meet Him” i.e., until death. “because they broke
that (covenant with Allaah) which they had promised to Him and because they
used to tell lies.” 

In conclusion: beware of making vows, and if you do not make
vows you will be fine. Do not oblige yourselves to do things that Allaah has
not obliged you to do except as the result of your own actions. If a person
has a sick loved one, let him say, “O Allaah, heal him.” If he wants to sit
an exam, let him say, “O Allaah, grant me success,” because some students,
when they find their studies hard and are afraid that they will fail,
promise Allaah that they will do various acts of worship if they pass the
exams. Then when they pass the exams, they start to ask questions and come
to the scholars asking for a way out from the vow, but there is no way out
and no other option than to fulfil the vow. End quote.