A SteamID object is an object, not a string containing the 64-bit SteamID. See info here from the SteamCommunity docs.
The SteamID returned when you retrieve the user's profile from their vanity URL is indeed correct:
> const SteamID = require('steamid')
> let sid = new SteamID()
> sid.universe = 1
> sid.type = 1
> sid.instance = 1
> sid.accountid = 117631762
> sid.getSteamID64()
'76561198077897490'
If you have the SteamID e.g. from passport-steam, then this is how you'd call getSteamUser:
const SteamCommunity = require('steamcommunity');
let community = new SteamCommunity();
community.getSteamUser(new SteamCommunity.SteamID('76561198077897490'), (err, user) => {
// . . .
});
That said, if all you're doing with node-steamcommunity is retrieving users' profile details, you're far better off using the WebAPI for that.
Fixed it, apparently I've been initializing a new SteamCommunity object in my main file. Thanks anyway, explaining this to someone really cleared my mind up