Is it a risky affair to book a flight from Udon to Chiang Mai at 18.10  the same day after you pick up the visa at Vientiane?

I would say it's doable but risky.

It should not be a problem it you are at the consular section at 1400 to pickup your passport. Others have done it.

I did it 2 times, it shouldn't be very risky, depends also what time you get your visa, how much you are willing to pay for the many different transfers, if you want to wait for other people to share the minivan or the taxi, if it's high/low season.

I'd do it again. It can be risky if you are short on money, if you get your visa at the very end, if there's a special event in the city (traffic jam).

Its possible. I was at the Udon airport two weeks ago and met a person who just returned from Vientiane and waited for the flight to Chiang Mai.