Reading then suffered a major setback on 30 minutes when Dave Edwards was sent off for a second yellow card, the Wales international clattering into Bjarnason.
Although Paul Clement's side survived unscathed until the break, Villa went on to take the lead shortly after the restart.
Villa's second came just after the hour, Snodgrass once again the provider with a delivery which was nodded home by Conor Hourihane.
Reading struggled to cope with their opponents' attacking endeavour and goal number three duly came on 70 minutes, Scott Hogan robbing the ball off Mannone and slotting into an empty net.
ASTON VILLA (4-1-4-1): Johnstone; Elmohamady, Chester, Terry, Taylor; Bjarnason; Snodgrass (Onomah 66'), Grealish (Whelan 75'), Hourihane, Adomah (Kodjia 66'); Hogan
READING (4-2-3-1): Mannone; Gunter, Blackett, Moore, Ilori; Edwards, Evans (Kelly 65'); Bacuna, Aluko (Martin 73'), Barrow; Smith (Clement 46')