$I have a 30-year-old Vivitar 283 flash, complete system with all the
$attachments which I used for many years with my Canon film cameras.
$Still works great. I don't dare use the flash with my 300D because I
$understand the high trigger voltage on these old flash heads could fry
$the digital camera.

  The botzilla listing of flash trigger voltages shows that 283s are
all over the map, from under 6V for some newer ones to as high as 600V
for some older ones!  It sounds like your old one is likely not safe
to use with your 300D.

$                    Is the Wein Safe-Sync attachment (about $50) a
$workable solution to use this old flash

  Yes.

$                                         or would I be better off
$putting the money toward a new, dedicated Canon flash?

  Your call.  The 300D uses E-TTL metering, which proved a bit
troublesome for a lot of DSLR users (and which explains why subsequent
models used E-TTL II metering), so you may actually get more
consistent exposure from your old flash.  But you won't get the features
of a dedicated unit, such as AF assist and high-speed sync.  So it
depends on what's important to you.

------------------------------------------------------------------
    Say hi to my cat -- http://www.stevedunn.ca/photos/toby/

I haven't tried it, but since it made for the purpose you describe, I
think it should work just fine.

But I would still like to suggest an alternative:

I have the same Vivitar 283 (one of the all-time great flash guns),
and I have adopted a slightly different solution to use it safely
with a new DSLR: Fire it via one of those cheap radio slave triggers
you can buy on eBay for around $25.

This cost you only half of the Wein Safe-Sync, and the flexibility
wireless triggering gives you is, a bonus:

More about these triggers here:
  http://hannemyr.com/photo/gt301b.html

I for one am not very damned happy with the e-ttl 300D + 550EX flash
system.  I offer this only to let you know that it's not all roses and
bon bons with the official canon solution.  

I've not yet tested if the e-ttl II system in the 350D and 400D
improves upon this.  

In reply, usually someone points me to the big bible of EOS flash
goodies which I've already read and acted on accordingly, but I still
am not nearly as happy as I was back in the Elan film world of the EZ
series flash and A-TTL.  

I'm hoping, however a body upgrade to E-TTL II might improve matters.  

So in short, if your vivitar has a thyristor, and you can get it to
work with an accessory, it might even be nice to shoot with in certain
scenarios than e-ttl dedicated flashes.

Best Regards,
--
Todd H.  
http://www.toddh.net/

It should be fine, yes.

Depends on the pricing & your own tastes, but $50 is a lot cheaper
than a new flashgun. You might try Googling for discount prices on new
& used EX-420s in your area.