Qitzur Shulchan Arukh – 67:1
סִימָן סז – הִלְכוֹת נְדָרִים וּשְׁבוּעוֹת
67: Laws of Vows and Oaths
Do not get in the habit of [making] vows [about objects; see next sei’f]. Whomever makes a vow, it is as though he built an altar on a high place at a time when altars on high place are prohibited [i.e. ever since the building of the first beis hamiqdash]. And whomever fulfills it, it is as though he offered on it a qorban, where he would be culpable because it was slaughtered outside [the beis hamiqdash]. For it is better that he ask [a beis din] about his vow and they annul it for him.
This is for the rest of vows, but vows making something sacred, it is a mitzvah to fulfill them, as it says “I will pay my vows to Hashem”, and he should only ask about them in a difficult situation.
This comparison is interesting. A qorban offered outside the beis hamiqdash is still called a qorban — which literally means “a closeness-causing thing”. However, it is prohibited because Judaism seeks holiness through structure, and thus a religious experience that violates halakhah is still punishable.
Many rishonim discuss this point when exploring the reason for the deaths of two of Aharon’s sons, Nadav and Avihu. They offered a “foreign fire” before Hashem, and died for it. But how? By Hashem’s fire taking them as an offering; and their death is described as “though My holy ones, I shall be sanctified”. Both sides of the dichotomy between the value of a religious experience and the destructiveness of not following the halachic regimen are acknowledged.
The comparison therefore affirms the religious content of the vow. It is trying to get religion by making an extrahalachic practice or prohibition binding on oneself that is problematic.