Can I use Vim instead of the atom based editor?
Is there a cost to being connected? If I’m understanding this correctly you have to pay for API calls/messaging which make the Photon a pay-us-monthly option compared to the Raspberry PI 3. It’s fairly easy to write REST API servers.
Yes, you can use any editor you like. There are several options for compiling and uploading code. There’s a web-browser based system, an IDE for Windows and Mac (Atom-based), a command-line cloud compiler (excellent with vim or other external editor, for Linux, Windows and Mac), and a system using a cross-compiler for ARM processors.
There is no charge for Photon usage. The Photon uses Wi-Fi, and you can use local TCP/IP communications on your own network, the Particle cloud (no charge) or other Internet-based services.
The usage charges you see are for the the Electron, which is a 2G/3G cellular modem device.