Most sources that posit a link between Vader and The Lightning admit that said link is based on speculation. This is the best summary I was able to find:
Darth Vader: The Lightning In The Fighting Devil Dogs
George Lucas once said, “With both Star Wars and Raiders, I started out by asking myself ‘Gee, when I was a kid what did I really like?’” The answer was that he loved the old serial adventures of the 1930s.
One of the earliest costumed supervillains in cinema was The Lightning, who appeared in the 12-chapter serial The Fighting Devil Dogs (1938). The Lightning wore a black leather costume, gauntlets, a helmet, and a cape, and it’s been speculated that these are all elements Lucas borrowed for Darth Vader, although Lucas has never actually confirmed that. Both Vader and The Lightning also had minions who wore all-white armor and helmets.
The Lightning, however, was not a mouth breather like Vader. That originated from artist Ralph McQuarrie, who was tasked with creating conceptual drawings of characters prior to the principle filming of 1977′s Star Wars Episode IV: A New Hope. In an early draft of the script, Vader was supposed to transfer from the Star Cruiser to Leia’s rebel ship via the vacuum of outer space, so McQuarrie drew a respirator on Vader’s mask. “George loved it,” McQuarrie said, and the respirator and the creepy, labored breathing became a permanent fixture of the Sith Lord.
This article quotes another piece, a 1981 article in the New York Times:
''With both 'Star Wars' and 'Raiders,' I started out by asking myself 'Gee, when I was a kid what did I really like?' '' Mr. Lucas recently explained. He liked the derring-do of the serials, and the unbeatable courage of their characters, not to mention the 30's settings. ''Practically every movie star of the 30's has one movie like this, be it Alan Ladd or Clark Gable or whoever - playing a soldier of fortune in a leather jacket and that kind of hat,'' Mr. Lucas said, referring to Mr. Ford's snap-brim. ''That's a favorite period of mine, but it was more the character we were after than the period, although they're obviously both rooted in the same ground. I took that character and put him in outer space and it worked just fine - not the same character exactly, but the same concept in terms of story and entertainment value.''