The answer to the first question, about the volume numbers: that's relatively easy. The volume number refers to which version of the series that had the name "Legion of Super-Heroes" and has nothing to do with reboots, creative team, etc.
Volume 1 (v1) = an early 1970's reprint series
Volume 2 (v2) = early to mid 80's. The original Superboy title from the 40's became "Superboy and the Legion of Super-Heroes" with issue 231, and then it changed again to "Legion of Super-Heroes" with 259.
Volume 3 (v3) = mid to late 80's. When the previous Legion title became "Tales of the Legion" with issue 313, the name was reused. This series was written almost entirely by Paul Levitz.
Volume 4 (v4) = When Levitz left, the series was cancelled and resumed with a new number 1 issue. Same title, now volume 4. This started with Giffen and the Bierbaums Five Years Later and ended with Zero Hour... except it didn't. After the reboot, the title continued the same numbering up through issue 125.
Volume 5 (v5) = the current series, the fifth book to have the title "Legion of Super-Heroes".
See? Relatively easy.
The other question, about the reprints, I've got a Legion reprint page at
http://lsh.0catch.com/lsh/lshrepr.htm that lists all of the Legion stories that have been reprinted. The Archives are chronologically reprinting everything, so it's easy to see (in the right-hand column) what went into which Archive book.