I am planning a trip to Pak Chong and either hit the vineyards and Khao Yai national park there.

Can anyone tell me where I can get on those minivans there. I know for a fact that I can get to Hua Hin and Kanchanaburi from around the hospital at Victory Monument but I am not sure about Pak Chong.

As nobody's responded to your post sassoc, I guess nobody knows. I'd be inclined to go back to the minivans near Victory Monument and ask the drivers, I'm sure they'll be able to point you in the right direction.
By the way, after you do the trip we'd appreciate a JBR on the vineyards.

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You're only young once, but you can stay immature forever.

sure not a problem. I have contacted a few vineyards already who suggested renting a car from bkk but I am not very keen on it given my bad experience in Phuket

I had a look around for you and found this out:

All trains going from Bangkok to Ubon Ratchathani stop at Pak Chong. Departures at Bangkok’s Hua Lamphong Station more than 5 times daily trip. Fare (excluding surcharges for rapid or express trains and aircon) is 36 Baht for 3rd class, 74 Baht for 2nd class and 146 Baht for 1st class. The rail distance is 180km. The journey maybe takes 3.5 hours.

You also can take bus from Bangkok to Pak Chong, which leaves from the Northern Bus Terminal(khampengpet rd). An air-conditioned bus is usually around 100 baht and takes two and a half to three and a half hours, to Pak Chong. Buses are suppose to run every 30 min.

I'd be inclined to take the train, hope this helps.

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You're only young once, but you can stay immature forever.

Thanks Bob, you have been really helpful, I guess if i do not get any info on the minivans, I guess the bus or the train it is.