Você pode usar join e getline . De :h join
join({list} [, {sep}]) *join()*
Join the items in {list} together into one String.
When {sep} is specified it is put in between the items. If
{sep} is omitted a single space is used.
Note that {sep} is not added at the end. You might want to
add it there too: >
let lines = join(mylist, "\n") . "\n"
< String items are used as-is. |Lists| and |Dictionaries| are
converted into a string like with |string()|.
The opposite function is |split()|.
e de :h getline
getline({lnum} [, {end}])
Without {end} the result is a String, which is line {lnum}
from the current buffer. Example: >
getline(1)
...
When {end} is given the result is a |List| where each item is
a line from the current buffer in the range {lnum} to {end},
including line {end}.
{end} is used in the same way as {lnum}.
Non-existing lines are silently omitted.
When {end} is before {lnum} an empty |List| is returned.
Example: >
:let start = line('.')
:let end = search("^$") - 1
:let lines = getline(start, end)
Colocando tudo isso junto em um exemplo útil:
:let buff=join(getline(1, '$'), "\n")