Hi,
The requirements to move from CSP to RBD/EGL on z/VSE are the same as for other environments from a migration perspective. Please take a look at the CSP to EGL migration guide at this link. VSE was not supported in CSP V4.1 which these guides use at the basis, but you should be able to get the general idea.
https://www.ibm.com/developerworks/community/wikis/home?lang=en#!/wiki/W75c8733d99bb_4d55_9ee8_4dbc8c56ebee/page/Cross%20System%20Product%20v4.1%20Migration%20to%20EGL
At an overview, you must migrate using this flow CSP->VAGen V4.5->RBD/EGL. Because CSP on VSE was the older interpretive version of CSP, you will need to be aware of the CSP migration considerations in going from interpretive to COBOL Gen. These can be found in the CSP to VAGen Migration manuals.
You can use any level of RBD/EGL and given RBD 9.1.1 is the latest, I would suggest you use this. To generate COBOL to execute on VSE, you also need to purchase the VSE COBOL Generation Feature (additional to the RBD base license). And of course, you need the COBOL Runtime that allows the EGL COBOL to run. This is installed on the host and this product is called the Rational COBOL Runtime for z/VSE V7.5.1 (Product ID - 5648-F66).
For overall system requirements, you can look at the RBD for VSE Generation Feature reference. The link below is from the RBD Knowledge Center. It also has a link to "developing" for VSE. You asked about the minimum, but I know RBD genned programs have also been executed on z/VSE V5.2 as well.
http://www-01.ibm.com/support/knowledgecenter/SSMQ79_9.1.1/com.ibm.egl.nav.doc/topics/vsegen_reference.html
Take a look at these docs and see if they provide the background you need.
markevans