How long can wegeners go undiagnosed? Can a patirnt have vague symptoms (still be able to funcion) for months before a diagnosis is made?

Yes! Delayed diagnosis can be caused by "vague" symptoms; mild disease at onset; the fact that wegener's is quite uncommon for an autoimmune disease. This rheumatic immune problem is best dealt with by a specialist in rheumatology.
Happens all the time. It's a rare illness and it usually get missed for months to years until the person becomes gravely sick. One young lady in my town eventually lost half her face because her personal physician treated it as "acne" for several years. I hope that if you're a Wegener's sufferer, you'll be able to comply with treatment which has turned was was a nearly-always-fatal illness into something manageable.