i confirm that i really dont find how to change the vsep in a block :

new texmacs document, i create a centered block, i adjust in (dont know exactly the english terms) focus/breaklines/centered

then i type something in the case of my block, say a, then new paragprah, then b :

––––

|a  |

|b  |

––––

then i selct my text (a and b) and i go to format/paragraph, no item seems to change the vsep, and from one document we are working on, the change of "pragrapah format/basic/vsep) makes sytematically TEXamcs crash, though the crash does not occur when doing the same on a blank document as described.