US copyright laws by way of the Library of Congress are very specific about what protections are offered to a copyright owner. Even though Youtube may not go after, or police music that is not a current hit, that doesn't mean the original copyright owner may not ask Youtube to ban the video, or the channel owner themselves.
Even if you are not profiting form the video, and/or the music, you are still in violation of a copyrighted item -- music, picture, video clip, etc. ... . by the illegal act of copying it without the owner's permission. This is all looked on as over-saturating a potential demographic, an area, region or market that could have an impact on the owner's ability to profit from their own work. In other words, if you own something, but people are putting it out there for free, it will be hard for you to profit from it later.
Because of this everything from record labels, to film studios will aggressively (like with an army of lawyers) go after copyright violators of their work. There are numerous stories of Disney suing daycare centers for having a mural of their characters to even individuals with tattoos of Disney copyrighted images.
Another reason for this is because of the legal nature of Copyright trials. If an owner allows one entity to use their work unchallenged -- say a T-Shirt vendor, but sues a Daycare center, a (Federal) judge will ask why did they allow one enterprise to use their work, but not another? This means if they do not aggressively protect their products all the time, future copyright cases are then weakened.
To legally obtain permission to a copyrighted sound track a vloger needs to acquire two Rights - Master Rights, generally obtained through the recored label, and the Synch Rights obtained by the musician/copyright holder. Both can range from free, to cheap to incredibly expensive.
In the past I obtained a the Festival (a combination of both) Rights to the song I, Lucifer by the band The Real Tuesday Weld at a cost of $1 USD for a single year. This was due to the owner's appreciation with how I politely asked them, but also their support of the short film I used it for. Currently, I'm seeking both the Master and Synch rights to a U2 song for another short film I've made.
Keeping my fingers crossed.
Finding Royalty free music is another good option.
In the past I would go to Myspace to look for musicians. Many often describe themselves as not having been signed to a record label. If you like their music, contact them and ask for permission to use their track. Many are thrilled to do so because it gives them exposure. I produce a half-hour public access TV show in Philadelphia. For each episode I try to find music from independent musicians to play over the ending credits. The majority of them I've found on Myspace.
Wuf