Taylor Swift and Marcus Mumford Perform at Las Vegas Eras Concert

Taylor Swift continues to blow fans away with one of the most well-received concert tours in years ... and this time a Mumford gave her an assist!

Taylor took her Eras Tour to Vegas Saturday night and the Allegiant Stadium was packed to the gills.

During her long set, she brought up a surprise guest -- Marcus Mumford.  The 2 performed "Cowboy Like Me,' with Taylor taking the lead and Marcus doing backup.

The fans were rabid, waiting more than 3 hours in line to buy some Swift merch. As for ticket prices ... well, an obstructed view seat on the secondary market went for north of $2k. Some of the premium tickets were topping $20k!!!

The concert, in which Taylor performs for hours without a break, has drawn high praise from the likes of former NFL star J.J. Watt, who was blown away at her kickoff concert in Glendale (aka Swift City) concert in Arizona.