Can my employer see what I am doing if I use a VPN?

Can I be tracked if I use VPN at work WIFI?

When a VPN service encrypts my network traffic, does the ISP know that I am using that VPN?

Let's say I have the "private" network address of 10. How can I be tracked by ISPs? How does a server on a private network work? Let's say that this server is 10.2 and has ip 10.3. If I use a VPN connection one of my devices and change the address of this device to 10.4, the server knows that the traffic is going through a VPN tunnel, but what can be done? It doesn't change it's IP to 10. The only things that change are the headers on the packets, right? How can an ISP distinguish between my device that has the private network IP of 10.1, and another device that has the private IP of 10.2, since they both have the same IP inside the network?
You're mistaken on a few points. In your "private" network, there is no distinction between addresses. All network traffic is simply labeled as private by IPTables and all IP traffic is allowed through it.

In a proper VPN, IP traffic changes IP to get back to your origin. The VPN software (a client) changes the source IP address to the gateway IP of the gateway at which the traffic is being sent. On top of that, the VPN software also changes the protocol of the traffic to one that matches its own protocol, so that it can do all sorts of cool stuff like encrypt the traffic and hide its true origin. On top of that, since your system is behind the VPN, it must also be NATed, so your machine will appear as 10.1 inside the network behind the VPN.

The ISP then can't tell what your original IP address was from the public IP address, it's just some unknown packet flowing through its network.