Are all entries to the Van Gogh Museum timed entries? Or can you show up, with or without an advanced ticket or Museumkaart and take your chances?
It's fine to show up without tickets. I did it in 2012. However, be prepared to wait in a line that could be very long
Well worth the wait though. Be sure to see Vincent's final painting Crows in a wheat field. Excellent !
If you want to skip any possible ticket-buying line, you can book online when you're more committed to your arrival time, like the night before.
You can show up and take your chances, but I would not recommend it. We went without Museumkaart or advance tickets in June 2015 and the line was very long (an hour or more I'm guessing). We went back to the Museumplein (1/2 way between the Van Gogh Museum and the Rijksmuseum, near some foods stands) and there is a small ticket office. This is a 5 minute walk from the Van Gogh Museum. You can buy advance tickets there for a number of attractions, including the Van Gogh Museum. We bought tickets for 15 minutes after the time of our purchase, walked over to the Van Gogh Museum and walked right in. My point is advance tickets would be a good idea, whether online, with Museumkaart, or some other way. Van Gogh Museum is so wonderful - his art obviously, correspondence with his brother providing insights into his deteriorating mental state, and much more. Enjoy!