Can anyone be rational and not vegan?

The word 'rational' is typically used to refer to a property of a belief system that leads to increased map territory convergence, or of a planning system that leads to increased degree of goal achievement.

The question of whether or not to be vegan instead mostly looks like a question of how much moral weight to put on various classes of entities, which doesn't seem like it's deeply related to either of those properties. There's probably a better adjective to use.

The very structure of the article suggests somebody wrote the bottom line first. I strongly doubt a rational agent examining the intersection of {ecology, economy, nutrition, morality} would arrive at veganism as the optimal solution.

Of course anyone can.

My thinking is different enough from the norm that I can't apply super-rationality, and as a single individual my individual efforts in eating or not eating meat have approximately 0% effect on animal suffering. Plus, as I mention here and there, my moral system is based around me being a selfish bastard (just not a /stupid/ selfish bastard, and a selfish bastard interested in the long-term fulfillment of my values, which tends to lead towards nearly the same behaviour of a rational altruist), and I have values I have a stronger wish to be fulfilled than eliminating non-sophont suffering (like spending my time trying to think of ways to ensure the continued existence of sophonts).

Are we sure that going vegan is the most rational solution to the problem? If we had a magical button that would instill a value in all human beings at the same time, would vegan be the best to install to solve the problem?
Since "going vegan" usually hand-waves the solution to a massive coordination problem, couldn't we just extract the hand-wave and apply it to something which is more effective?

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Since rationality requires going paleo, Yes.

No. Nobody can be purely rational - we just don't know how to construct minds that are free of bias, or if it's even possible.

Thus, we don't know if vegan diet is what a rational mind would choose.

[edited to clarify that this mixes 2 points] 1) Nobody is rational, so nobody is rational and non-vegan. Nobody is rational and vegan for that matter.
2) More importantly, we don't even know what the math looks like for the calculation of what to eat. Especially for farmed animals, where you first have to solve the existence-value question (is it better to exist and then be eaten, or not to exist at all).

Given the huge effect of the meat an dairy industry on the environment, if humanity doesn't go vegan soon, we will probably go extinct. Can a rational agent behave in ways that advance its own destruction? Isn't self-preservation a necessary condition for maximizing any other goal?