Not played the game through to know the stage you are talking about, and ive not got the GECK in front of me, but:
You can fire up the geck and look for the name of the quest in the quest section of the object window. If its a main quest, then it will probably start off something like NVMQ and then a number, but as I say - no GECK so I cant confirm that. You can select each quest and check the description/name box, which will have its full title. Once you know which quest you need, check the quest stages section. There will be a series of entries, probably ranging from 0, 5, and ending in 100. These represent different stages of a quest, from beginning to completion.
You want to figure out which stage you want to set yourself too, so check the descriptions. If its the start, it should be fairly obvious.
Now, you need to look up the formid of the quest. Check in the object window, and look at the name of the quest e.g. NVMQchimp. Now look to the right of that, there is a collapsed box represented by two close together vertical lines. Normally this field is hidden because its not useful in general modding, but you need the code for use in the console. Expand the column to reveal the formid, which will be a long series of numbers e.g. 000013FB etc.
Make a note of the formid, and the stage of the quest you want to set yourself to in order to fix our bug (e.g. if its the start of the quest, it may well be stage 0 or stage 5 or something.
Open new vegas, then open the console. Type :
setstage
So for example:
setstage 000013FB 5
Hope that helps somewhat. If someone has a GECK they can look up the code / quest stage for you probably.