Thanks for the questions. I know I have used both terms in regard to c 603 and solitary eremitical vocations over the years. I am not sure I have ever defined them specifically and especially I am sure I have never distinguished the two of them in the way you are asking about. Definitely my bad! The questions are good and helpful, so again, thanks.
A fallback vocation is a term sometimes used for cc 603-604 vocations by chancery and other personnel who don't believe these are valid vocations, either because they are not good fit for the individual seeking consecration under these canons, or because the chancery does not esteem or believe in the vocations more generally (that is, no matter who is seeking consecration in these ways). It also has the sense that one would not seek out consecration under these canons unless one failed at religious life and can't really handle that "failure". In such a case the person uses either c 603 or c 604 as a "fallback" vocation.
When these canons were newly promulgated (October 1983) and for some years thereafter the Archdiocese (and Region) of Los Angeles refused to consecrate or profess and consecrate anyone at all under either canon reasoning that these persons were merely attempting to get consecrated in whatever way they could while unsuitable for life in a religious community. The essential problem with such a position is that while a diocese (and candidate) ought certainly to be aware of the temptation to use these canons in this way and discern whether or not they are doing so, one cannot simply conclude the vocations themselves are fallback vocations, or, correlatively, that religious life is the only valid form of consecrated life. By refusing to profess and/or consecrate anyone at all under these canons the Archdiocese of LA proclaimed both vocations per se to be invalid. More, in doing so it is to reject the Universal Church's perception and insight in this matter and judges negatively a diocese's capacity to carefully or accurately discern such vocations.
The additional problem with LA's (former?) position is that it fails or failed to regard the fact that if one is called to a rare and little-known or understood vocation like solitary eremitical life, they are apt to try more common and well-known vocational options first and only in time and with experience, discover the vocational path they are truly called to. This is not a matter of settling for a fallback option, but rather of needing the time and other resources and opportunities necessary to truly discern one's true vocation. Related to this is the fact that simply because of one's own vocational maturation one may grow into eremitical life (for instance) over time. (Monks often saw eremitical life as the summit of monastic life.) To label c 603 vocations "fallback vocations" is to prematurely rule out of court these kinds of vocational situations as well.While in some ways the term "stopgap vocations" could be used synonymously for fallback vocations, I use it on this blog to mean a vocation or path to profession and consecration which itself has no canonical framework or process of implementation (or which some find too onerous to adhere to). So, for instance, some people determine that going through the ordinary canonical process for forming a community is too burdensome and time-consuming for them; it is seen to have too many procedural hoops to jump through and (often) to require too-assiduous supervision. Instead, they seek to be professed and consecrated under c 603, and once that is done, they seek to create communities of "hermits" also professed/consecrated under c 603 despite the fact that c 603 was not meant to be used in this way. Let me be clear, these folks have not discerned a solitary eremitical vocation and in fact, do not feel called to one, but are using c 603 as a stopgap. Others do not want to live in community (nor are they really hermits) but wish to be considered religious (mainly to dress the part, style themselves as Sister or Brother, reserve Eucharist in their own places, find validation in the Church, etc) and they seek to "stop the gap" in canon law regarding professing individual religious (the Episcopal Church has a canon professing individuals as solitary religious who may or may not be hermits; the Roman Catholic Church does not).
The root of both of these terms involves a failure to esteem the nature and charism of the vocations defined and rendered canonical by canons 603 and 604. Sometimes the diocese is culpable in this way, sometimes the individual seeking consecration is culpable in this way and sometimes both are culpable in this way, but the roots are the same and include a failure to actually discern the vocation involved. There are a number of variations on the examples I have given which make vocations either fallback or stopgap. Authentic vocations are neither!!
I hope this is helpful.