I don't see why the fitness training has to be separate from the Wing Chun training.
You blokes must do power training at some stage in your class right?
Someone who knows how to feed the pads properly with focus mitts ,Thai pads or kick shield can make the partner work very hard indeed.
You can do simple stuff like ten kicks of your choice on the kick shield , do the other leg , then hand the shield over and change roles.
Alternate that with fast punches on the kick shield in between sets of kick reps and keep up the pace for 15 minutes and it will be quite demanding power training.
Other options can be "chasing the pad" , one partner holds the kick shield and moves around presenting the pad at different positions and angles for various leg and arm strikes.
The person chasing must use the correct kick depending on the range and position of the pad , like medium heel kick for waist height , if the pad holder holds it low on the front of the leg then the chaser does a low heel kick , if on the side of the leg he does a hook kick with the shin , you get the idea.
The pad holder can also get sneaky and swing the kick shield at the other guys head randomly if he starts to drop his hands.
This drill can be quite exhausting.
Another one that can have you hurling your guts out onto floor if you do it with intensity is this one , have one partner start chain punching the chest of the other partner , on your signal the person getting punched starts dancing around doing their best to get away from the puncher.
The person chain punching must try to adjust their footwork and keep in range with their punching , striving to keep proper contact with their punches.
As well as improving their fitness it will also help in their mobility and distancing of tracking a moving object, for the more delicate souls they could wear a protective vest and the puncher wear mma style gloves.
For advanced students you can have the person designated as a mobile target actually start throwing random punches back so that the Wing Chun guy whilst in the process of chain punching also has to defend.
To get the most out of this the person playing the non Wing Chun role really has to do their best to be as mobile as possible moving randomly in all different directions , trying to side step etc.
Really just trying to make it very hard for the Wing Chun guy and make him continually adjust to stay in punching range and correct his positioning.
This type of stuff will work on aerobic and anaerobic fitness while still being specific to the skills needed in Wing Chun.