Delete space-expanded "tab" in Vim with one keystroke?

You can use Ctrl+D to back up one tab stop. This actually moves the whole line to the left one tab stop; Ctrl+T does the same thing to the right. Note that these keystrokes only work in Insert mode (use << and >> for the equivalent in Command mode).

"tl;dr: set tabstop=4 softtabstop=-1 shiftwidth=0 expandtab

short form: set ts=4 sts=-1 sw=0 et

Explanation

If you set softtabstop (or sts) to -1 it will automatically behave the
same as tabstop (ts), which will save you some hassle if you change tabbing a lot. Setting shiftwidth (sw) to 0 should effectively make
that the same as tabstop as well.

In Detail

shiftwidth sw

Number of spaces to use for each step of (auto)indent. Used for cindent, >>, <<, etc.
When zero the tabstop value will be used.

tabstop ts

Number of spaces that a in the file counts for. Also see
:retab command, and
softtabstop option.

softtabstop sts

Number of spaces that a Tab counts for while performing editing
operations, like inserting a Tab or using BS. It
feels like Tabs are being inserted, while in fact a mix of spaces
and s are used. This is useful to keep the tabs is setting at its
standard value, while being able to edit like it is set to sts. When
sts is negative, the value of shiftwidth is used. This will save you
some hassle if you change tabstops a lot. When expandtab is not set, the
number of spaces is minimized by using Tabs.

expandtab et

In Insert mode: Use the appropriate number of spaces to insert a .
Spaces are used in indents with the > and < commands
and when autoindent is on. To insert a real tab when expandtab is on,
use Ctrl-V Tab. See also
:retab"

easiest way is <<, repeat with a .

"Have you checked using the expand tabs setting in your VIM?

:set noet

I usually prefer keeping the tabs on while working on the files.
When it is required, I replace the tabs to 4 spaces or as many as required.

:%s/<ctrl+V><tab>/ /g
----

As a small bonus, your source file is shorter by 3 chars per tab :-)"

I don't think there's a way. Once the tab key is expanded to four spaces, vim has no way of knowing they were ever a tab.