But it can't be helped. On this ultralight level, the only thing that even attempt to fulfill at least two roles while make it cheap is VTOL. Regardless how good it is, non-VTOL are expected be inferior to VTOLs at this level. To compare with 15 tons VTOLs those ground vehicles are should be at least 25 to 40 tons, I think. Only after then they may compete with VTOLs by its speed.
That said, although it's obvious that OP wants the VTOL, but if other than VTOL then what I think are;
Diabolicus Line Tank (Prototype)
Mass: 15 tons
Movement Type: Tracked
Power Plant: 75 Fusion
Cruising Speed: 54 kph
Maximum Speed: 86.4 kph
Armor: Hardened
Armament:
1 ER Medium Laser
1 Small Pulse Laser
1 Small X-Pulse Laser
Manufacturer: Unknown
Primary Factory: Unknown
Communication System: Unknown
Targeting & Tracking System: Unknown
Introduction Year: 3145
Tech Rating/Availability: E/X-X-X-E
Cost: 367,425 C-bills
Type: Diabolicus Line Tank
Technology Base: Inner Sphere (Advanced)
Movement Type: Tracked
Tonnage: 15
Battle Value: 439
Equipment Mass
Internal Structure 1.5
Engine 75 Fusion 3
Cruising MP: 5
Flank MP: 8
Heat Sinks: 10 0
Control Equipment: 1.0
Power Amplifier: 0.0
Turret: 0.5
Armor Factor (Hardened) 48 6
Internal Armor
Structure Value
Front 2 11
R/L Side 2/2 10/10
Rear 2 6
Turret 2 11
Weapons
and Ammo Location Tonnage
Small X-Pulse Laser Turret 1.0
Small Pulse Laser Turret 1.0
ER Medium Laser Turret 1.0
Unlike VTOLs, which can enjoy its speed and able to fly over the terrain, tracked vehicles have lesser demand for the same level of protection on its back. Also it can take hardened armor too. Although it is not fast, but that's all I could without choose a VTOL instead. And it can withstand an AC/20 hit, which is not bad. All the weapons are energy based so it never fear for the ammunition explosion.
Diabolicus Line Tank (Production)
Mass: 15 tons
Movement Type: Tracked
Power Plant: 75 Fusion
Cruising Speed: 54 kph
Maximum Speed: 86.4 kph
Armor: Heavy Ferro-Fibrous
Armament:
1 ER Medium Laser
1 Small Pulse Laser
1 Small X-Pulse Laser
Manufacturer: Unknown
Primary Factory: Unknown
Communication System: Unknown
Targeting & Tracking System: Unknown
Introduction Year: 3145
Tech Rating/Availability: E/X-X-X-D
Cost: 416,300 C-bills
Type: Diabolicus Line Tank
Technology Base: Inner Sphere (Standard)
Movement Type: Tracked
Tonnage: 15
Battle Value: 451
Equipment Mass
Internal Structure 1.5
Engine 75 Fusion 3
Cruising MP: 5
Flank MP: 8
Heat Sinks: 10 0
Control Equipment: 1.0
Power Amplifier: 0.0
Turret: 0.5
Armor Factor (Heavy Ferro) 89 4.5
Internal Armor
Structure Value
Front 2 20
R/L Side 2/2 20/20
Rear 2 9
Turret 2 20
Weapons
and Ammo Location Tonnage
Trailer Hitch Rear 0.0
Trailer Hitch Front 0.0
Small X-Pulse Laser Turret 1.0
Recon Camera Turret 0.5
Small Pulse Laser Turret 1.0
ER Medium Laser Turret 1.0
Targeting Computer BD 1.0
But, if you could leave the rear armor weak, it does not have to be hardened, is it?
Diabolicus Line Tank (Insulated)
Mass: 15 tons
Movement Type: Tracked
Power Plant: 75 Fusion
Cruising Speed: 54 kph
Maximum Speed: 86.4 kph
Armor: Heavy Ferro-Fibrous
Armament:
2 ER Medium Laser
1 Small X-Pulse Laser
Manufacturer: Unknown
Primary Factory: Unknown
Communication System: Unknown
Targeting & Tracking System: Unknown
Introduction Year: 3145
Tech Rating/Availability: E/X-X-X-F
Cost: 477,250 C-bills
Type: Diabolicus Line Tank
Technology Base: Inner Sphere (Experimental)
Movement Type: Tracked
Tonnage: 15
Battle Value: 487
Equipment Mass
Internal Structure 1.5
Engine 75 Fusion 3
Cruising MP: 5
Flank MP: 8
Heat Sinks: 10 0
Control Equipment: 1.0
Power Amplifier: 0.0
Turret: 0.5
Armor Factor (Heavy Ferro) 89 4.5
Internal Armor
Structure Value
Front 2 20
R/L Side 2/2 20/20
Rear 2 9
Turret 2 20
Weapons
and Ammo Location Tonnage
Trailer Hitch Rear 0.0
Trailer Hitch Front 0.0
Small X-Pulse Laser Turret 1.0
3 Laser Insulator Turret 1.5
2 ER Medium Laser Turret 2.0
If you want two ER medium lasers but don't want to give up the anti infantry weapon then it's the option. Since laser insulator have very small chance to malfunction, it won't be a problem for the most times.
Diabolicus Line Tank (Ram)
Mass: 15 tons
Movement Type: Tracked
Power Plant: 75 Fusion
Cruising Speed: 54 kph
Maximum Speed: 86.4 kph
Armor: Hardened
Armament:
3 Small X-Pulse Laser
Manufacturer: Unknown
Primary Factory: Unknown
Communication System: Unknown
Targeting & Tracking System: Unknown
Introduction Year: 3145
Tech Rating/Availability: E/X-X-X-E
Cost: 328,325 C-bills
Type: Diabolicus Line Tank
Technology Base: Inner Sphere (Advanced)
Movement Type: Tracked
Tonnage: 15
Battle Value: 395
Equipment Mass
Internal Structure 1.5
Engine 75 Fusion 3
Cruising MP: 5
Flank MP: 8
Heat Sinks: 10 0
Control Equipment: 1.0
Power Amplifier: 0.0
Turret: 0.5
Armor Factor (Hardened) 48 6
Internal Armor
Structure Value
Front 2 10
R/L Side 2/2 10/10
Rear 2 8
Turret 2 10
Weapons
and Ammo Location Tonnage
Trailer Hitch Rear 0.0
Trailer Hitch Front 0.0
3 Small X-Pulse Laser Turret 3.0
That is a variant for the fight in the urban area, largely based on prototype.