Problem switching from MPLS to SDWAN

My company is trying to switch over from our MPLS (Windstream) to an SDWAN (VeloCloud).

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We have twenty locations, one of them being a datacenter that houses our servers.

Each of our sites has access to the other sites through the MPLS routing.

Each of our sites have multiple /24 networks. 10.10.1.0/24 10.10.2.0/24 10.10.3.0/24 etc.

DHCP is being provided for these sites by the Windstream boxes.

(sometimes multiple WS boxes competing with competing DHCP servers with different networks)

We have static IP devices on various networks at different locations.

All of our switches throughout the company are in layer 2 mode.

There are no VLANs. Many of our older Switches don't have VLANs functions.

Our domain controllers, DNS, and email server, shared folders, are at the datacenter.

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We've been working with an outside vendor to configure the SDWAN

They are telling us that they cant do multiple networks on a single interface with these SDWAN (VeloCloud - Edge 510) boxes.

They are saying that the only way to do this would be to create VLANs.

But that's not really possible without replacing a lot of hardware

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Is there another way that this can be done? maybe configuring one switch to put between the SDWAN box and the rest of each locations network to passthrough VLANs somehow from these other layer 2 mode switches at each site? Or is replacing most of our switches and configuring VLANs our only option?