Vegas currently has 26M in cap space for the 2018 trade deadline, and 58M in projected cap space for the 18-19 season (
https://www.capfriendly.com/teams/goldenknights). Orpik for Neal adds $0.5M, so cap hit is a non-issue, from Vegas' perspective. Each played about 17:40 per game last year. Orpik did so on a team he just helped win back to back president's trophies. Those go to the best team in the league over an 82 game season. Orpik started a majority of his shifts in the defensive zone, and wound up with a plus/minus of +32, while landing 181 body checks and blocking 132 shots (
https://www.hockey-reference.com/players/o/orpikbr01.html). Neal managed to score 23 goals, five of them on the powerplay, and posted a -10 and 82 hits (
https://www.hockey-reference.com/players/n/nealja01.html), for the 8th place team in the Western Conference. Then, in the playoffs, he put up nine points in 22 games, and went -8. I don't think the Caps open up a hole in their excellent defense to add a somewhat productive winger who is a big defensive liability.
If injuries open up cap space, I could imagine the Caps renting a veteran like Neil at the trade deadline to play third line minutes, but they don't have any room for him on their top power play unit, so they're not likely to overpay. Maybe a 2nd or a 3rd round pick for the pending UFA, if Vegas is unable to extend his contract. Probably another team is more desperate for a couple goals, and offers more. The Caps' GM, MacLellan, tends to announce what he's planning to do. This year he says he's getting his team younger and faster. Vegas has a lot of players who fit that mould. I could see MacLellan pursuing Miller, McNabb, Marchesault... and paying market value to make a lasting addition to their club.
The only way the Caps trade Orpik for a declining forward on a pending UFA contract is if they somehow believe they can keep Carlson and add Tavares. Even in that wild dream, Bowey is still counted on to become an NHL regular, in order to make the salary cap work, and is untradeable due to a lack of depth at RHD. Vegas can afford to bank a bunch of LHD's as a scarce resource to make long-term advantageous trades. Washington wants to win right away, before Ovechkin gets any older, so they need to keep a good balance of left and right handed blue liners.