I’ve used many panels in the past, but I’ve never used HSphere, and have a few questions…
Do I get a VPS account with my reseller account? If so what is it used for? Linux or Windows hosting?
Yes you can get a VPS on linux, it is for linux apps.
How is it possible to sell Linux/Windows in one account?
It is not in one end user account, it is in one reseller account, sub accounts can be linux or windows.
Do I get two panel accounts, one for windows and one for Linux, or is it integrated with HSphere?
It is integrated into one interface.
I use ModernBill, I assume I would continue using it, and that you don’t have any integrated billing solutions that comes with your reseller accounts?
Hsphere has a billing and automation platform built in, but you can continue to use MB if you wish and setup the accounts manually, however keep in mind that bandwidth overages, upgrades etc will need to be manually charged by you if NOT using hsphere.
What does it mean by user accounts? I use cPanel now, and I can create unlimited user accounts, including email, ftp etc…
A user account is a control panel that manages an account with as many domains as you allow in it, it is true multiple domain hosting where I read other places Cpanel is really just addon domains 
Do you offer Windows VPS?
Yes, but not at a reseller account level, the support levels just don’t allow for it.
What is the cost for VPS for resellers?
Linux VPS is included up to some limits as listed on plans, windows pricing is as listed.
How long does it take to provision a VPS account for a client?
Linux VPS accounts can take up to 2 hours. Windows accounts take 24 hours to setup and as said before are not included in any way in the reseller accounts.
On the VPS accounts, is all server security managed by Jodohost? Minus apps I install ofcourse…
No, we do not manage security in any fashion on the VPS accounts, it is the users responsibility, IRC is not allowed, DDoS attacks and spambots being run result in immediate termination of the VPS.