Check whether "Virto_Alerts_Customizer_Job" is working in Central Admin. If it is not working, try to recreate the job using the following SharePoint Management Shell (run as administrator) script:

$job = get-sptimerjob -identity "Virto_Alerts_Curtomizer_Job" -webapplication http://yourwebapplicationurl
if ($job -ne $null) {$job.Delete()}
$jobName="Virto_Alerts_Customizer_Job"
[reflection.assembly]::LoadWithPartialName("Virto.SharePoint.AlertSendJob")
[Microsoft.SharePoint.SPSchedule] $schedule = [Microsoft.SharePoint.SPSchedule]::FromString("Every 1 minutes between 0 and 59")
$job = New-Object -typename "Virto.SharePoint.AlertSendJob.SendEmailJob" (Get-SPWebApplication http://yourwebapplicationurl)
$job.Name = $jobName
$job.Title = $jobName
$job.Schedule = $schedule
$job.Update($true)
Get-SPTimerJob -Identity $jobName -webapplication http://yourwebapplicationurl | Start-SPTimerJob