There are indeed short and long vowel sounds in Spanish, but they are not phonemic, i. e. they are not contrastive. Vowels tend to become longer when stressed; this is very common across languages. Wikipedia says "stressed syllables can be up to 50% longer in duration than non-stressed syllables" in Spanish.
Since Japanese has a vowel quantity contrast but does not have phonemic stress, using one to suggest the other sounds right: it amounts, in fact, to forcing the same correlation between length and stress that is found in Spanish.
The process whereby vowel quantity was lost in Vulgar Latin must have taken quite a long time, but as far as I know Old Spanish already had no trace of vowel quantity. Wikipedia says "Length confusions seem to have begun in unstressed vowels, but they were soon generalized." This is to be expected, since unstressed syllables are usually most prone to loss of phonemic distinctions (neutralization).
In Classical Latin, actual length was not the only difference between so-called long and short vowels: as it happens in many other languages, long vowels were also tense and short vowels were also lax. Mistakes noted by grammarians show that this was so: people tended to mix up short e and i and short o and u (long and short a could not be distinguished by tenseness; /a/ lost its quantity contrast when the rest of the vowels did).
In the end the tense-lax contrast won the game and people started distinguishing vowels only by tenseness and only in stressed syllables. Later the system was simplified even more by conflating (in some branches of Romance) lax i with tense e and lax u with tense o.
The above is how Italian and Portuguese got their seven vowels. Spanish went further by diphthongizing the lax vowels in stressed syllables (/e/→/ie/, /o/→/ue/) and merging them with the tense ones elsewhere, ending up with the current five.
Right now phonemic vowel length exists in a few Romance languages, but in every case it is a new development.