Is a neder against doing a prohibited behavior a neder?

Question

If one makes a neder not to do something issur derabbanan, if they break it, are they liable for the neder?

Answer 1

Yes the Neder works to forbid something which is already prohibited miDerabanan Gemara Shavuos 20b:

כיום שנהרג בו גדליה בן אחיקם איצטריך ליה סלקא דעתך אמינא כיון דכי לא נדר נמי אסור כי נדר נמי לא הויא עליה איסור והאי לאו מיתפיס בנדר הוא קמ"ל
If one makes a Neder like a Neder he already made on the Fast of Gedalia not to eat, one might have thought it is already prohibitted to eat and therefore the Neder is invalid, so the Halcha tells us the Neder works on top of the Pre-existing prohibition.

Rashi even extends this rule that the Neder is valid even on pre-existing Issurei deoraisa (e.g eating on Yom Kippur see tosfos below) rejecting the text of the the Gemara which says the Neder works only in the case of Issurei Derabanan :
ולא גרסינן כיון דמדרבנן הוא דאסור כי נדר חייל עלי' איסו' בלא יחל לא גרסינן לה להא דהא א"נ מדאורייתא הוא אמרינן (לקמן כה.) הנדרי' חלים על דבר מצו' כדבר הרשות

But Tosfos (see @aleychem quote of Shulchan aruch YD 215,5 who agrees to Tosfos) limits the Neder to work only on Issurei Derabanan like the text Rashi Rejected because an Issur deoraita cannot take place on a pre-existing Issurei deoraita:
אבל אם נדר לקיים מצוה כגון שאמר קונם שלא אוכל ביוה"כ או שלא אוכל נבילות וטריפות לא חייל דאין איסור חל על איסור ולהכי איצטריך טעמא דמדרבנן

Answer 2

This is my resolve, completely for the sake of discussion, and it's not to be trusted in reality

First, Neder, unlike Shvua, gives property to an item, it's not an obligation on an action (I will do...I will not do...).

Assuming you meant really a Neder (e.g "This cheese-and-chicken sandwich (Issur DeRabanan) is forbidden to me like eating a sacrifice").

Shulchan Aruch Yoree Dea paragraph 215 5 says that a Neder can be applied on Mitzvat Ase (positive command, like sit in a succa during the festival), but not on Mitzvat Lo Tase (negative command, like don't eat pork).

Since all Issur DeRabanan are actually Mitzvat Ase (ועשית ככל אשר יורוך), I would guess that the Neder applies.