Delete everything between two brackets in Vim, including newlines

You can simply do:

ca[[]<Esc>

or:

ca][]<Esc>

See :help text-objects.

With your cursor on the first opening bracket ([), press V, followed by %. This will select the block which you then can join J, followed by di[.

Select the lines in visual mode (v) and use J to remove newlines. Then use di[.

Or if there are many lines, di[ first, after which you move the cursor to top line and then J. This will potentially leave a space between the brackets which has to be removed with x.