1. Have you ever seen VHS cassettes with this well-known "VHS" stamp and under it the inscription: "PAL SECAM" ?
From what I know, in some of the countries there was a situation of recording the signal in PAL, but converting the material for television to SECAM (television broadcast in such a system).
2. I don't know how the situation was in the case of VHS recordings. Do you know ?
3. Even if it was so that VHS cassettes were also recorded in PAL, then converted to SECAM and distributed in this way, capturing in PAL would probably provide the best color quality for such material than if it were captured in SECAM ? Would have to check it yourself in practice ?
[The VHS cassettes, which I have from the nineties and the twenty-first century, the stickers that are stuck on the cassettes have a known stamp: VHS, and under it the inscription: "PAL SECAM". Only "VHS" is engraved on the cassettes (plastic).]
4. What is the difference between D and K systems: Is it for PAL or is it for SECAM ?
As far as I know, D and K systems were used here(Poland) for television (whether for SECAM or PAL; for television: SECAM was used until 1993/1994 and later PAL). The differences I know [for TV] are apparently: D used in the VHF band, and K in the UHF band (for PAL; I don't know for SECAM differences).
5. Which PAL (or SECAM) system should I use to capture the signal from a VHS cassette [Poland] ?
Theoretically, there should be some connection with these systems between television and VHS recordings (especially private ones). Perhaps it also depended on the recording camera which one had (the equipment) - from which country and which VHS tape was used.