[vi] How can I delete multiple lines in vi?

You can delete multiple(range) lines if you know the line numbers:

:[start_line_no],[end_line_no]d

Note: d stands for delete

where,
start_line_no is the beginning line no you want to delete and end_line_no is the ending line no you want to delete. The lines between the start and end, including start and end will be deleted.

Eg:

:45,101d

The lines between 45 and 101 including 45 and 101 will be deleted.