In current versions it is
varnames <- vapply(vars, deparse2, " ")[-1L]
and deparse2() is a slightly enhanced version of the above
function, again with 'width.cutoff = 500'
*BUT* if you read help(deparse) you will learn that 500 is the
upper bound allowed currently. (and yes, one could consider
increasing that as it has been unchanged in R since the very
beginning (I have checked R version 0.49 from 1997).
On the other hand, deparse2 (and your older code above) do paste
all the parts together via collapse = " " so I don't see
quite yet ...