When viewing illegal web sites using a hotspot connection from my cell phone is it essential to have VPN for both my cell and my tablet?

If you want you ISP (including your cell company) to be suspicious of just what you are doing, then use a VPN. As far as an ISP is concerned, the VPN can be like waving a red flag saying you are doing something suspicious.

When you use a VPN such as you imply with your question, then you will be using a proxy server along with many other people who are using the same VPN and Proxy Server. The connection beyond the proxy server is not protected and information at that point is no longer masked by the VPN. If the authorities want to check for possible illegal activity then checking what is going into and out the proxy site is a good place for them to start. You home IP address will be visible in the VPN connection between the proxy and they can get your details from your ISP.

Don't access illegal sites. You could have your equipment confiscated, receive a fine a criminal record or even a prison sentence.

It is not essentials, HOWEVER!!! I would highly recommend it to avoid detection of any government beauras snooping around looking for people to bust. I would highly recommend you get multiple VPNs and run at least 2 at once. Remember there is always the risk of still being arrested.