[OT] pcvt, need help on vi (and/or tr) (and/or g77)

[Alfred Breull <puma_(_at_)_hannover_(_dot_)_sgh-net_(_dot_)_de>]

2.8, i386, pcvt, ksh (console)

German keyboard (with the lines scon -o, kcon -m de in rc.local).
However, I've problems with special signs in vi (and tr).


On vi:

How do you get umlauts (ä,ö,ü) and ß in vi ?
(instead of \xe4, \xf6, \xfc and \xfd)

How do you get special characters, i.e. greek sigma (msdos ascii 
oct 344 or dec 228) in vi ? 

Backgroud:
My fortran source code contains the lines
       real asum
       ...
       write (6,600) asum
       ...
 600   format (1x,'dec228 a(i,j)= ',1f7.3)

with dec228 standing for the capital greek sigma (sum sign), ascii
decimal 228. In fact, this shows the capital greek sigma in msdos 
(sorry, I don't know how to create the letter in this mail program).
However, I don't know how to do this in vi.

I've tried to create special characters with tr. However, I can't
even change ctrl-A (\001) into ctrl-C (\003) by

tr '[\001]' '[\003]' 
\001
 
tr '\001' '\003' 
^A

Each time, I just receive an empty line. :(

I've been looking thru 'Unix in a Nutshell', 'Unix Power Tools'
with no success. I have an ascii table, showing all signs from 
dec 000 to dec 255. However, I can't create these signs in vi.

So, please, send me to a related man page, url or such.
-Alf