Hey, Seppo,

I just finish my dinner and saw yoru post here... Great now we all can try this vegetarian version "Vietnamese fish sauce". Some information for you here... I wouldn't sat that Thai fish sauce isn't as good, though. I will just say that it's "different". Whenever possible, I try to use Thai version for Thai food and Vietnamese one for Vietnanmese cuisine... :)
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From "The Classic Cuisine of Vietnam", Bach Ngo and Gloria Zimmerman,
Barron's, 1979.

Vietnamese Fish Sauce (Nuoc Mam)

Fish sauce is to Vietnamese cooking what salt is to Western and soy
sauce to Chinese cooking. It is included in practically all recipes.
Prepared from fresh anchovies and salt, layered in huge wooden
barrels, the manufacture of fish sauce is a major industry. The
factories are located along the coast to assure the freshness of the
fish to be processed. Fermentation is started once a year, during the
fishing season. After about 3 months in the barrel, liquid drips from
an open spigot, to be poured back into the top of the barrel. After
about 6 months the fish sauce is produced.

The first draining is the very best fish sauce, lighter in color and
perfectly clear. [Kinda like "Extra Virgin" fish sauce. S.C.] It is
relatively expensive and is reserved for table use. The second and
third drainings yield a fish sauce of lower quality and lower cost
for general- purpose cooking. The two towns most noted for their
fish sauce are Phu Quoc and Phan Thiet. Phu Quoc produces the best
fish sauce, some of which is exported. On the label, the "nhi"
signifies the highest quality. When fish sauce manufactured in
Vietnam is not available, that of Thailand or Hong Kong is quite
acceptable. Philippine or Chinese fish sauce will not be
satisfactory. For table use and available in all Oriental groceries
is Squid Brand Fish Sauce, the best one on the market. Whatever
brand, look for the "Ca Com" on the label, which means that only
anchovies were used++an indication of the highest quality for table
use.
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Oyster sauce is a different sauce all together. It's flavor mainly comes from oysters and is a Chinese (Cantonese) creation. There is vegetarian oyster sauce available (by Lee Kum Kee, again.) Here is the information I got from the Lee Kum Kee site.
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Premium Oyster Flavored Sauce

Premium Oyster Flavored Sauce is made from the best quality oysters with that rich oyster taste. Made according to the original recipe created by the founder of Lee Kum Kee in 1888, it is an essential Chinese cooking sauce. It can be used for dipping, marinating, stir-frying and cooking a wide range of Asian dishes.

Panda Brand Oyster Flavored Sauce

Panda Brand Oyster Flavored Sauce is second in quality only to Lee Kum Kee's Premium Oyster Flavored Sauce. This sauce is prepared from quality oyster extracts. Its robust flavor, rich color and fine texture make it a great all-purpose sauce for every day cooking.

Choy Sun Oyster Flavored Sauce

Choy Sun Oyster Flavored Sauce contains no MSG. It has a balanced flavor and smooth texture; which makes this sauce perfect for stir-frying, marinating, gravy-making and dipping.

Vegetarian Oyster Flavored Sauce

Vegetarian Oyster Flavored Sauce imitates the flavor of oyster sauce by using the essence of shiitake mushrooms. It is excellent for braising or as a dip, enhancing the flavor of vegetarian dishes.
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For more reicpes using (premium/normal/vegetarian) oyster sauce, check out their page:

http://www(dot)lkk(dot)com/e_oyster_rec(dot)htmlI hoep Betsy is OK with me posting their URL...

WARNING: THIS SITE IS USUALLY SLOW FOR SOME REASON!