This old model has one disadvantage: a high trigger voltage, more than 260 volts, some samples over 280. Such a high voltage is deadly for digital cameras that conform to ISO 10330 standard and withstand 24V. Let alone Canon cameras that ignore standards and don't accept more than 6V!
There are some solutions, like Wein Safe syncs, or photo-slaves, or DiY adapters (http://repairfaq.cis.upenn.edu/sam/strbfaq.htm#strbtoc). All of these solutions are external accessories, which makes them not so handy and let you with the risk of forgetting to use them.
The really safe solution is simply to include the (simple) protection circuit into the device itself, and lower the sync voltage to under 6V, be it on the shoe, the remote cable, or the Vivitar PC cord. This is important for people who had to replace a worn out or broken shoe with a metallic one. They are exposed, when touching the shoe off camera, to electric shocks, not really dangerous, but surely unpleasant.
Step 1: Schematics
A pdf file is available here http://repairfaq.cis.upenn.edu/sam/zpaofu1.pdf , probably more legible than the poor jpeg hereafter.
Step 2: The Guts
We want to include the adapter inside the flash, so we'll have to open it. There is already a good instructable which shows us how to do so. You'l find it there:
How to disassemble a Vivitar 283https://www.instructables.com/id/How-to-disassemble-a-Vivitar-283/
We'll also unscrew the photocell/remote socket and remove the battery compartment, to get some room for the soldering iron.
The upper (moving) part, the flash tube housing and the trigger circuit don't need to be disassembled and they can stay in place.
The adapter circuit will take place electrically between the remote socket and the trigger input, physically between the trigger circuit and the battery compartment.
Step 3: Wires to Cut
The blue wire (#2) is the "common", negative pole, leading to the center contact of the remote socket. We'll cut it, and later electrically connect the two ends together again
The remaining length (photo 2) of the wire from the remote socket is not sufficient to allow us, when reassembling the flash, to comfortably put back the two screws of the socket. We'll have to solder 3cm of wire, insulated with a piece of heat shrinkable sheath.
On the other side, we'll add a new wire, the pink one on the second picture.
Step 4: Assembling the Circuit
The two pictures, components and solder sides, should be clear enough.
On the solder side, the four 1 megohm resistors are SMD (Surface Mounting Devices) 1206 types,
The diodes (one small signal, one Zener) are SMD SOD80 types. The PMLL4148 (=1N4148, black ring) is soldered just between the Gate and Anode 1 pins of the triac (MAC97A8 or equivalent).
The pad under each resistor is removed with the tip of the hot soldering iron and a reasonably violent pressure.
Step 5: Connections Inside
The two ends of the blue one, common, are connected again together, as previously announced.
The part coming from the socket should be extended, which is not done on this picture
The pink wire now leads to the socket a reduced trigger voltage (<5V).
From the socket, when the PC cord is not plugged in, the trigger voltage is lead to the remote socket, and from there to the flash shoe, provided the sensor is plugged into the socket.
Before re-assemblig the flash, we'll turn the board with the component (insulated) side against the main PCB, to avoid any unintended (and possibly disastrous) contact.