I'm a med student so I'll put my 5 cents in from that perspective. In general, doctor's are going to be concerned about the bottom line of the patient's health. If a patient is vegan, then this will not usually be a concern as long as the diet is nutritionally sound (meaning, the patient's specific implementation of veganism is good, not veganism in general). The down-side is that you may need to explain why your diet is ok to your doctor. Doctors aren't trained in alternative diets and they very well may not know anything about it or just know what they've heard from others (and we all know how that goes...).
Having said that, doctor's are just regular people with a lot of education. So, nutrition aside, you're going to find the same mix of opinions that you would anywhere on veganism/AR. Will those opinions enter the clinic? Sometimes, it depends on who the doc is. Some people are just narrow-minded jerks, but a lot of doctors know how to keep their personal opinions to themselves when it comes to patient care (like they should!).
As far as getting blood work on a vegan at a checkup, I think that is warranted. Especially with a new patient or a patient who is new to veganism. It is always better to know that your body is getting all the right nutrients than to assume it.
I looked at that link on SDN. That's probably a good example of how you will find the usual random mix of opinions on veganism in the medical community. And since us vegans are an extreme minority in the US, I'd say this is pretty normal to be mostly on the omnie-side.
As for my personal opinions/experiences. I would certainly have no problem with a vegan patient as long as he/she was knowledgeable and responsible enough to do it correctly (especially important for pregnant women and children) and I have never had any problems when telling my doctors that I am vegan.