Firstly, I already have an Private VPN, even though seedbox provides whole package that includes VPN/storage/server. In this case, are there any option to use my vpn to use with seedbox?

As has been said most seedboxes will allow third party VPN's. Seedboxes.cc provides a VPN, but using their VPN does count against the data up/down threshold. I will say that as long as you have a trusted VPN you're good to go with any seedbox you choose (From my experience).

Secondly, even before seeing the benefit of seedbox, I would have to sign up with private tracker that are open. I believe a lot of private tracker do not allow to vpn is that right?

To a degree that is correct. Some do and some don't. It depends on the tracker. A lot of the trackers will not allow you "signup" with the VPN. But once you have signed up you can use your VPN without problems. Most of them don't have issues with this, but not all are ok with it. Read the FAQ and reviews of the tracker, and ask around here. You'll get some good answers. I know IPT and PrivateHD hasn't had problems with my VPN, not yet.

I am confused on a particular area, how are the private tracker able to track my ratio when I will be in anonymous most of the time.
Though it is "anonymous" they track the torrent. So when you download the torrent it is associated with your account. Once you upload the torrent reports back what you're doing. If you have seedbox and the tracker has an option for making that known, you just enter that you have one and the IP address/host.
Also, it should be noted that as long as you are consistent and check your ratio you can alert staff if there are problems. If you get a ban for any reason you can talk with them and explain the situation of using a seedbox.

I have noticed there was a forum section for seedbox sales, So when I am purchasing one, does this mean I getting a dedicated server for myself. Can you guys recommend me as to which one to get in terms of reliablity/lowcost/speed. The seedbox server/storage is usually a shared slot... much like a webserver. There are dedicated slots/servers but they are generally more expensive. Which one is best, is up for debate. RapidSeed is decent, but price for storage is not the best, IMO. However RapidSeed allows you to run handbrake which you can compress your files right there on the seedbox, and run plex as well.

Seedboxes.cc is nice, I like it alot. The prices are a bit steep, but you get a lot of nice applications (sickrage, sickbeared, seedbucket, plex and so on). I also use seedbox.io. I enjoy their price for the year, but they are offering only a seedbox, no plex, no apps of any kind outside of the torrent client. But I can download on it with great speeds and have a backup of all my files.

I hope this helps. Also, to help you out more; http://seedboxgui.de/ is a great review site that helped me make my choices when I was first considering a seedbox.

cheers!