You are correct about s_vi being the visitor ID, this should stay the same for every visit (at least we hope so!). The visit ID (the visit number part) is calculated after the hit is collected so it isn't available through dynamic variables.
I think you can get what you are after using a combination of the visitor ID and visit number from the getVisitNum implementation plugin, http://microsite.omniture.com/t2/help/en_US/sc/implement/#getVisitNum
Instead of using the s_vi dynamic variable, I'd suggest reading the s_vi cookie directly, then reading the s_fid fallback cookie, and then doing something else if this fails (either don't set the variable so it reports as "None", or set it to something custom). That way you won't get an empty ID for visitors who aren't cookied.
var id = s.c_r('s_vi') != "" ? s.c_r('s_vi') : s.c_r('s_fid'); if (id != "") { var dt = new Date(); s.eVar1 = id + "|" + dt.getUTCMonth() + "|" + dt.getUTCFullYear() + "|" + s.getVisitNum(); } else { //do something else, user does not accept any cookies... }This should result in something like:
4E39CEC3FE367A9A-178C79544A5B6A15|2|2014|1
4E39CEC3FE367A9A-178C79544A5B6A15|2|2014|2
You'll need to customize this more for your needs but it should get you going down the right path.