What is the ruling on someone who vowed to fast for two consecutive months, but failed to do so?
He is obliged to fulfill his vow by fasting for two consecutive months, but if he fails to do so then, it remains a debt on him until he either fulfills his vow, or dies. In case of his death, his guardian is supposed to fast on his behalf, or donate food from his estate in return for fasting, but if the guardian fails to do so then, it is all in the Hands of Allah.
Is it permissible for the guardian to give Sadaqa (voluntary charity) from the money of the orphans?
It is impermissible for the guardian to donate from the money of the orphans because he is entrusted with the safekeeping of that money, and is prohibited from donating it.
I made a vow "If the Israeli troops retreated out of Gaza strip I will buy a cake from a certain bakery" and it happened. Am I obliged to buy from the same bakery?
You are obliged to buy from the same bakery. And Allah Knows Best.
What is the ruling on obtaining a house through the King Abdullah II project (Decent Housing for a Decent Living), knowing that we do not own a house, our financial situation is moderate, our current rental costs are high, and we are eight people living in the household?
If owning the apartment/house is done through the same institution/project then it is permissible, but if this took place through usurious banks then it is impermissible, since dealing with usurious transactions is unlawful. And Allah Knows Best.