Reply$---$ Virtual Log File$($VLF$)$ are the part of internal transaction of log file. The transaction log file are the part of database which is used for recovery purpose. When a database change is done like update, insert it first log the change into the transaction log$,$ VLF are the chunks of transaction log files which is maintained internally by the database. Is means the database mainly SQL Server divides the transaction log file in smaller log file which is called virtual log file.

VLF$\text{'}$s are discussed because of how critical they are to checkpoints.

At the time of recovery process if the VLF are too many $,$ then it reserves a huge amount of space for a small amount of write $,$ so for maintaining all the VLF will also slow the performance of the database. Ans also at the time of recovery it will take a huge time as the number of VLF are high.

Too few VLFs also leads slow the performance because when the database process trying to write in the VLFs it has to search in which VLFs it has to write or which VLF it had to overwrite which leads to a slow performance impact.