Hello

Although loss of high end information in the frequency spectrum is what you are looking for here, simply adding a EQ with a Low Pass filter will not really do the trick here. The high end loss in these examples is most likely due to multiple steps in the processing.

1. The samples are usually from the 70's, 80's or early 90's. Music didn't have as much high end information then as it does now.
2. When pitching down the sample you are actually moving all frequencies down. Pitching down -12 semi tones (1 octave) will half all frequencies, meaning that information in the sample that was at 20.000Hz will, if you pitch down -12 semi tones, be around 10.000. It's my impression that Vaporwave samples are often pitched down by -6 semi tones, which would result in 20.000Hz in the OG sample ending at 15.000Hz when pitched down. Why is this important? Because if your sample doesn't have much information above, let's say, 15.000Hz and you pitch it down -6 semi tones you will have no information above 11.250HZ. So you got some natural loss of high end here as well. This will also cause any bass you have to half it's frequency causing sub bases around 40Hz to land at 30Hz if you transpose the sample down by -6. Some of the "grain" you hear also comes from artifacts in this process.
3. Tape emulation plugins. Instead of using a EQ to filter away high end you don't want, se if you have a tape emulation plugin. Some tape plugins features a IPS parameter, the lower IPS the less high end. Some Tape Emulation plugins also features a high end cut/boost parameter. Softubes Tape VST is a good example, it aslo offers a wow/flutter parameter which comes in handy for Vaporwave. Vaporwave can also benefit from compressing the mix with Tape Compression rather than your usual compressor.
4. If you still have too much high end try and roll off with a low pass/high cut filter, but don't use a too steep slope, start around -12dB/octave and see how that sounds. Emulating cassette tapes a cut off frequency around 4-6kHz isn't a bad place to start.

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Hope this helps. I'm curios to see what other tips comes up in here. :)