Q:

How to make clear in the speech that a short English vowel is stressed? 

I'll try now to explain. 
Let us take the word Berlin (name of the capital of Germany).  It is pronounced [bɜːˈlɪn] or [bɝːˈlɪn] (I followed this link).  In any of two cases, the first vowel is long but unstressed, and the second one is short but stressed. 
I wonder how to pronounce correctly such words, how to evidentiate that the second vowel of the word carries stress but is short.  In Russian, we almost always make the stressed vowel two or even three times longer that any unstressed; it's the longest sound in the word.  In English, however, as I know, you cannot change the length of a vowel as you want.  It may lead to wrong pronunciation or even change of the word's meaning. 
I tried to ask a similar question at another site.  I think I didn't receive a very good answer there; all that I understood is that English unstressed vowels are diffuse.  I would appreciate a clearer answer. 

A:

In Russian, you do a lot more than make it longer ;)
Compare:

Write as: Pro to'go ko'torogo (lyubila)
Read as: Pra ta'vo ka'torava (lyubila)

In the case of Berlin: I suggest 2 changes to the sound:

make it longer (as you already suggested);
pronounce it louder (of course, louder as compared to the other sounds). 

It should sound like "blin / блин" (= pancake) but with detached "b": b-lin / b(a)-lin.  The quality of "i" is the same in both words.